Add instrumentation to the MessagePumpMac family of classes.

Add instrumentation to the MessagePumpMac family of classes.

This adds UMA-uploaded histograms of sampling intervals for key points of data:
* Total time spent in a CFRunLoop
* Total time waiting in CFRunLoop
* Total time servicing MessagePump::Delegate (the MessageLoop)
* The MessageLoop queue size and queueing delay before servicing

It adds 1 second sampling for 15 seconds at startup, only for the main thread
(NSApplication-based) run loops.

The data will be used to determine if adding scheduling-like behavior to the
MessagePump will result in more efficient processing of the MessageLoop work.

An unforunate side effect of this change is exposing another method on the
MessagePump::Delegate interface, but there does not appear to be a better way
to do this.

BUG=264886
R=jar@chromium.org, mark@chromium.org

Review URL: https://codereview.chromium.org/22911026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221427 0039d316-1c4b-4281-b951-d872f2087c98