Automatically adapt to faster/slower uploads of renderer histograms...

Automatically adapt to faster/slower uploads of renderer histograms

This replaces the current time based approach (chrome is given N seconds
to upload all renederer histograms) with an asynch callback approach
that waits until all renderers have responded (with their updates). It
uses a fall-back timer to ensure that a hung renderer won't delay things
forever as well.

This causes faster (and complete) updates in about:histograms as well
as generally assuring complete updates during UMA gatherings.

This code was contributed by Raman Tenneti in CL 42496
http://codereview.chromium.org/42496

bug=12850
r=raman

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=17123