Add class to parse chrome histograms.

Existing hw decode video tests need to observe the output of
chrome://histograms/<some_histogram> to tell if hw was used.

Every test was repeating this logic. Moving this responsibility into one
simple
class.

BUG=None.
TEST=ran all modified video tests.

Change-Id: I3526706b79c7fda8e2129fe2c2707ed27b7b7583
Reviewed-on: https://chromium-review.googlesource.com/219875
Reviewed-by: Rohit Makasana <rohitbm@chromium.org>
Tested-by: Mussa Kiroga <mussa@chromium.org>
Commit-Queue: Mussa Kiroga <mussa@chromium.org>
4 files changed