[ MimeHandlerView ] Add a new UMA bucket

This CL adds one more bucket to MimeHandlerViewUMATypes which counts the usage
of internal calls (from C++) to postMessage (used for printing).

TBR=wjmaclean@chromium.org

Bug: 659750
Change-Id: I08c5194012c97f0a603dd338e454b201dfafd1da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1576204Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652748}

extensions/common/guest_view/mime_handler_view_uma_types.h

@@ -36,7 +36,10 @@ class MimeHandlerViewUMATypes {
     // For recording postMessage to embedded MimeHandlerViews (e.g., MHVs not
     // created due to navigations to the resource).
     kPostMessageToEmbeddedMimeHandlerView = 10,
-    kMaxValue = kPostMessageToEmbeddedMimeHandlerView,
+    // For recording postMessage from internal APIs (includes 'print' messages
+    // full page MimeHandlerView).
+    kPostMessageInternal = 11,
+    kMaxValue = kPostMessageInternal,
   };
 
  private:

extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container_base.cc

@@ -276,6 +276,7 @@ void MimeHandlerViewContainerBase::PostMessageFromValue(
   v8::Isolate* isolate = v8::Isolate::GetCurrent();
   v8::HandleScope handle_scope(isolate);
   v8::Context::Scope context_scope(frame->MainWorldScriptContext());
+  RecordInteraction(UMAType::kPostMessageInternal);
   base::AutoReset<bool> avoid_recording_internal_messages(
       &should_report_internal_messages_, false);
   PostJavaScriptMessage(isolate,

tools/metrics/histograms/enums.xml

@@ -37507,6 +37507,9 @@ Called by update_use_counter_css.py.-->
     Any message posted to a MimeHandlerViewContainerBase that is embedded; i.e.,
     is not created as a result of navigating a frame to the resource.
   </int>
+  <int value="11" label="PostMessageInternal">
+    Emitted when an internal postMesasge to 'print' is issued from C++ code.
+  </int>
 </enum>
 
 <enum name="MissingStartType">