Switch the TCP reads on Windows to use non-blocking/non-async I/O.

chrome/renderer/page_load_histograms.cc

Index: chrome/renderer/page_load_histograms.cc
===================================================================
--- chrome/renderer/page_load_histograms.cc	(revision 161340)
+++ chrome/renderer/page_load_histograms.cc	(working copy)
@@ -928,6 +928,43 @@
     }
   }
 
+  // Histograms to determine if disabling overlapped TCP reads
+  // has an impact on PLT.
+  static const bool disable_overlapped_reads =
+      base::FieldTrialList::TrialExists("NBReadImpact");
+  if (disable_overlapped_reads) {
+    UMA_HISTOGRAM_ENUMERATION(
+        base::FieldTrial::MakeName("PLT.Abandoned", "NBReadImpact"),
+        abandoned_page ? 1 : 0, 2);
+    UMA_HISTOGRAM_ENUMERATION(
+        base::FieldTrial::MakeName("PLT.LoadType", "NBReadImpact"),
+        load_type, DocumentState::kLoadTypeMax);
+    switch (load_type) {
+      case DocumentState::NORMAL_LOAD:
+        PLT_HISTOGRAM(base::FieldTrial::MakeName(
+            "PLT.BeginToFinish_NormalLoad", "NBReadImpact"),
+            begin_to_finish_all_loads);
+        break;
+      case DocumentState::LINK_LOAD_NORMAL:
+        PLT_HISTOGRAM(base::FieldTrial::MakeName(
+            "PLT.BeginToFinish_LinkLoadNormal", "NBReadImpact"),
+            begin_to_finish_all_loads);
+        break;
+      case DocumentState::LINK_LOAD_RELOAD:
+        PLT_HISTOGRAM(base::FieldTrial::MakeName(
+            "PLT.BeginToFinish_LinkLoadReload", "NBReadImpact"),
+            begin_to_finish_all_loads);
+        break;
+      case DocumentState::LINK_LOAD_CACHE_STALE_OK:
+        PLT_HISTOGRAM(base::FieldTrial::MakeName(
+            "PLT.BeginToFinish_LinkLoadStaleOk", "NBReadImpact"),

[Ryan Sleevi, 2012/10/11 18:53:16]

Same comments elsewhere about over-abbreviation for "NBRead"

+            begin_to_finish_all_loads);
+        break;
+      default:
+        break;
+    }
+  }
+
   // Site isolation metrics.
   UMA_HISTOGRAM_COUNTS("SiteIsolation.PageLoadsWithCrossSiteFrameAccess",
                        cross_origin_access_count_);