Add UMA histograms to MHTML save operations.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index a9fa068a901edfad4ae901224cb3d05ddef89946..c66a968f3fd36fc8138d6dd6a3af2e105d046a8b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -40408,6 +40408,101 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram
+    name="PageSerialization.MhtmlGeneration.BrowserWaitForRendererTime.FrameTree"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Total time the browser process waited for all render processes to save their
+    respective frames while saving a page into MHTML.
+  </summary>
+</histogram>
+
+<histogram
+    name="PageSerialization.MhtmlGeneration.BrowserWaitForRendererTime.SingleFrame"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time the browser process waited on a single frame to be saved by a render
+    processes while saving a page into MHTML.
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.MhtmlGeneration.EncodingTime.SingleFrame"
+    units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time taken to encode into MTHML the fully serialized contents of a frame.
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.MhtmlGeneration.FullPageSavingTime"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>Time taken to save a page into an MHTML file.</summary>
+</histogram>
+
+<histogram
+    name="PageSerialization.MhtmlGeneration.RendererMainThreadTime.FrameTree"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent by the main threads of all involved render processes while saving
+    the frame tree of a page to MHTML.
+  </summary>
+</histogram>
+
+<histogram
+    name="PageSerialization.MhtmlGeneration.RendererMainThreadTime.SingleFrame"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent by the renderer main thread while saving one frame of a page to
+    MHTML.
+  </summary>
+</histogram>
+
+<histogram
+    name="PageSerialization.MhtmlGeneration.SerializationTime.SingleFrame"
+    units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time taken to fully serialize the contents of a frame, including HTML and
+    CSS and image resources.
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.MhtmlGeneration.WriteToDiskTime.SingleFrame"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent writing a frame's encoded MHTML data to the file on disk.
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.SerializationTime.CSSElement"
+    units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent serializing a CSS element (including embedded &quot;sub&quot;-CSS
+    and images).
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.SerializationTime.Html" units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time taken to generate HTML data from a frame's DOM and serialize it
+    (without sub-resources like CSS and images).
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.SerializationTime.ImageElement"
+    units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>Time spent serializing an image element.</summary>
+</histogram>
+
 <histogram name="Parser.AppendBytesDelay" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>