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..523fd5f312f64a5b134c1f44f8a86d2bb0b04e49 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -25514,14 +25514,6 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="Mist.SwitchResult" enum="MistSwitchResult">
-  <owner>benchan@chromium.org</owner>
-  <summary>
-    The result (e.g. success or the type of failure) of a modem interface switch
-    operation performed by mist on Chrome OS.
-  </summary>
-</histogram>

[Ilya Sherman, 2016/09/28 00:42:16]

nit: Please revert this removal -- I assume it's an accidental deletion, yeah?

[carlosk, 2016/09/28 17:02:44]

Gah! Thanks for noticing this one! My bad and reverted!

Funny fact: for one moment I thought this was really removed in ToT as
codesearch could not find any matches for Mist.SwitchResult. I have the
impression that it used to index this file and show its contents but it's not
the case anymore. :/

-
 <histogram name="MobileDownload.BytesDownloaded" units="KB">
   <owner>qinmin@chromium.org</owner>
   <summary>
@@ -40241,6 +40233,101 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram
+    name="PageSerialization.MhtmlGeneration.BrowserWaitForRendererTime.FrameTree"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Total time the browser process waited for all render processes to save their
+    respective frames while saving a page into MHTML.
+  </summary>
+</histogram>
+
+<histogram
+    name="PageSerialization.MhtmlGeneration.BrowserWaitForRendererTime.SingleFrame"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time the browser process waited on a single frame to be saved by a render
+    processes while saving a page into MHTML.
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.MhtmlGeneration.EncodingTime.SingleFrame"
+    units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time taken to encode into MTHML the fully serialized contents of a frame.
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.MhtmlGeneration.FullPageSavingTime"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>Time taken to save a page into an MHTML file.</summary>
+</histogram>
+
+<histogram
+    name="PageSerialization.MhtmlGeneration.RendererMainThreadTime.FrameTree"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent by the main threads of all involved render processes while saving
+    the frame tree of a page to MHTML.
+  </summary>
+</histogram>
+
+<histogram
+    name="PageSerialization.MhtmlGeneration.RendererMainThreadTime.SingleFrame"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent by the renderer main thread while saving one frame of a page to
+    MHTML.
+  </summary>
+</histogram>
+
+<histogram
+    name="PageSerialization.MhtmlGeneration.SerializationTime.SingleFrame"
+    units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time taken to fully serialize the contents of a frame, including HTML and
+    CSS and image resources.
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.MhtmlGeneration.WriteToDiskTime.SingleFrame"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent writing a frame's encoded MHTML data to the file on disk.
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.SerializationTime.CSSElement"
+    units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time spent serializing a CSS element (including embedded &quot;sub&quot;-CSS
+    and images).
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.SerializationTime.Html" units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time taken to generate HTML data from a frame's DOM and serialize it
+    (without sub-resources like CSS and images).
+  </summary>
+</histogram>
+
+<histogram name="PageSerialization.SerializationTime.ImageElement"
+    units="microseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>Time spent serializing an image element.</summary>
+</histogram>
+
 <histogram name="Parser.AppendBytesDelay" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>