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 224814a18431e6aa6824392aac58e2e62d33157c..20fa6be0470f4b8df7db8765f1b5d3527bb8598b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -34432,6 +34432,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.ResourceLoader.ExpectedContentSize.CorrespondsBodySize"
+    enum="ResourceLoaderExpectedContentSize">
+  <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. Recorded for each resource load.
+  </summary>
+</histogram>
+
+<histogram name="Net.ResourceLoader.ExpectedContentSize.IsKnown" enum="Boolean">
+  <owner>maksim.sisov@intel.com</owner>
+  <owner>mmenke@chromium.org</owner>
+  <summary>
+    Counts how many times expected content size is known in advance. Recorded
+    for each resource load.
+  </summary>
+</histogram>
+
 <histogram name="Net.ResourceLoader.InliningStatus"
     enum="ResourceLoaderInliningStatus">
   <owner>tzik@chromium.org</owner>
@@ -34457,6 +34476,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.ResourceLoader.TotalBodySize.LT_AsyncHandlerBufferSize"
+    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>
@@ -101852,6 +101881,12 @@ value.
   <int value="1" label="Has client"/>
 </enum>
 
+<enum name="ResourceLoaderExpectedContentSize" type="int">
+  <int value="0" label="CORRESPONDS_TO_RESPONSE_BODY"/>
+  <int value="1" label="LT_RESPONSE_BODY"/>
+  <int value="2" label="MT_RESPONSE_BODY"/>
+</enum>
+
 <enum name="ResourceLoaderInliningStatus" type="int">
   <int value="0" label="Applicable"/>
   <int value="1" label="EarlyAllocation"/>