Remove CompressibleStringImpl::s_isPageBackground

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 b5a898b6df1e8505268736b6a8fad01ac19d7137..b6bc92540685357bac8ec1eabbf82cbc3369ece6 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -59160,8 +59160,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <enum name="CompressibleStringCountType" type="int">
   <int value="0" label="Compressed in a background tab"/>
-  <int value="1" label="Decompressed in a background tab"/>
-  <int value="2" label="Decompressed in a foreground tab"/>
+  <int value="1" label="Decompressed in a background or a foreground tab"/>

[Ilya Sherman, 2016/01/28 02:08:03]

Generally speaking, we recommend only making backwards-compatible changes to
histograms.  However, for fairly new histograms, that don't yet have very much
data, it's ok to make a non-backwards-compatible change if you think there's not
enough data to potentially cause confusion.  (Keep in mind that old versions of
Chrome will always have users, so there will always be data trickling in from
users of any versions that shipped between when you first added this metric and
this current change to it.)

 </enum>
 
 <enum name="ComputeCurrentSigninStatus" type="int">