Group flakiness results by blink revision

This patch reorganizes the flakiness dashboard to group results by
blink revision so that regressions are easier to spot.

With patch:
http://head-and-shoulders.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=svg%2Fbatik%2Ffilters%2FfeTile.svg

Without patch:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=svg%2Fbatik%2Ffilters%2FfeTile.svg

This patch works by creating an array of the unique builds. In the
simplest case this array will have a blink revision corresponding to
each unique build, and each column in the flakiness results will
correspond to one of these blink revisions. Because a bot can run a
test multiple times for the same blink revision (e.g., if the chrome
version changed), blinkBuildRevisions can return multiple
entries with the same blink revision.

htmlForTestResults has been updated to use the new unique revisions.

To make the results easier to understand, we "interpolate" results
for revisions that were not tested. For example, if r123 passed and
r126 passed, r124 can be an interpolated result which probably passed.
These interpolated results use a lighter color.

NOTRY=true

Review URL: https://codereview.chromium.org/359533004

git-svn-id: svn://svn.chromium.org/blink/trunk@178388 bbb929c8-8fbe-4397-9dbb-9b2b20218538