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 db79a3ce5c1c80b78b99517e17a4c6ee11bfab69..305c06305892c4359e276b814f268c96ca99af33 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -25514,6 +25514,71 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram
+    name="MhtmlGeneration.AllRenderersMainThreadTimeForFrameTreeSerialization"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent by the main threads of all involved render processes while
+    serializing the frame tree of a page being saved to MHTML.
+  </summary>
+</histogram>
+
+<histogram name="MhtmlGeneration.BrowserWaitTimeForFrameSerialization"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time the browser process waited on a single frame serialization by a render
+    processes while saving a page into MHTML.
+  </summary>
+</histogram>
+
+<histogram name="MhtmlGeneration.BrowserWaitTimeForFrameTreeSerialization"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Total time the browser process waited on all frame serializations by their
+    respective render processes while saving a page into MHTML.
+  </summary>
+</histogram>
+
+<histogram name="MhtmlGeneration.FullFrameEncodingTime" units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time taken to encode into MTHML the full serialized contents of a frame.
+  </summary>
+</histogram>
+
+<histogram name="MhtmlGeneration.FullFrameSerializationTime"
+    units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time taken to serialize the full contents of a frame, including HTML and
+    resources.
+  </summary>
+</histogram>
+
+<histogram name="MhtmlGeneration.FullPageSavingTime" units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>The total time it took to save a page into an MHTML file.</summary>
+</histogram>
+
+<histogram name="MhtmlGeneration.RendererMainThreadTimeForFrameSerialization"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent by the renderer main thread while serializing one frame of a page
+    being saved to MHTML.
+  </summary>
+</histogram>
+
+<histogram name="MhtmlGeneration.WritingToDiskTime" units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent writing MHTML serialized data to the file on disk.
+  </summary>
+</histogram>
+
 <histogram name="Mist.SwitchResult" enum="MistSwitchResult">
   <owner>benchan@chromium.org</owner>
   <summary>
@@ -40241,6 +40306,27 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageSerialization.CSSSerializationTime" units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent serializing a top level CSS resource (including embedded
+    &quot;sub&quot; CSS and images).
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.HtmlSerializationTime" units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time taken to generate HTML from an exiting frame contents and serialize it
+    (without sub-resources).
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.ImageSerializationTime" units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>Time spent serializing an image resource.</summary>
+</histogram>
+
 <histogram name="Parser.AppendBytesDelay" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>