Enable WebVTT regions for runtime testing, updated tests and minor fixes

This patch removes all the ifdefs surrounding WebVTT regions and enables
the feature for testing in RuntimeEnabledFeatures, along with a couple
of bug fixes.

Tests were rebased wrongly (since the feature wasn't build by default),
so I updated the -expected and added two others that refer to the display
of regions and update TestExpectations to not skip these anymore.

Review URL: https://codereview.chromium.org/25798003

git-svn-id: svn://svn.chromium.org/blink/trunk@159784 bbb929c8-8fbe-4397-9dbb-9b2b20218538