Add UMA histograms to AsyncResourceHandler to track content size.

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 b8ed3e9575857bf9e225b2e331cd8ee86875f5cd..4818d8120a2545a05c8b62f3b8d494d25a5115fc 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -34733,6 +34733,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.ResourceLoader.ExpectedContentSizeResult"
+    enum="ResourceLoaderExpectedContentSizeResult">
+  <owner>maksim.sisov@intel.com</owner>
+  <owner>mmenke@chromium.org</owner>
+  <summary>
+    Records how many times expected content size equals/less/more than size of
+    read body/buffer or content size is unkown. Recorded for each resource load.
+  </summary>
+</histogram>
+
 <histogram name="Net.ResourceLoader.InliningStatus"
     enum="ResourceLoaderInliningStatus">
   <owner>tzik@chromium.org</owner>
@@ -34758,6 +34768,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.ResourceLoader.TotalBodySize.LT_AsyncHandlerBufferSize"

[jwd, 2017/01/19 21:21:36]

Are you going to remove this?

[maksims (do not use this acc), 2017/01/20 05:51:01]

Oh, sorry. Yes. thanks for pointing to it.

+    enum="Boolean">
+  <owner>maksim.sisov@intel.com</owner>
+  <owner>mmenke@chromium.org</owner>
+  <summary>
+    Counts how many times size of a loaded resource is less than allocated
+    buffer's size. Recorded for each resource load.
+  </summary>
+</histogram>
+
 <histogram name="Net.ResponseSizeByProcess.Browser" units="KB">
   <owner>juliatuttle@chromium.org</owner>
   <summary>
@@ -102746,6 +102766,15 @@ value.
   <int value="1" label="Has client"/>
 </enum>
 
+<enum name="ResourceLoaderExpectedContentSizeResult" type="int">
+  <int value="0" label="EQ_RESPONSE_BODY"/>
+  <int value="1" label="EQ_RESPONSE_BODY_GT_EQ_BUFFER_SIZE"/>
+  <int value="2" label="GT_EQ_BUFFER_SIZE"/>
+  <int value="3" label="LT_RESPONSE_BODY"/>
+  <int value="4" label="GT_RESPONSE_BODY"/>
+  <int value="5" label="UNKNOWN"/>
+</enum>
+
 <enum name="ResourceLoaderInliningStatus" type="int">
   <int value="0" label="Applicable"/>
   <int value="1" label="EarlyAllocation"/>