Don't find(':') twice in WebVTTParser::collectMetadataHeader

Since contains(':') is the equivalent of a find(':') - except it throws
away the result - and the result might be of interest later on, just
call find(':') once instead, and remove the call to contains(...)
(essentially open-coding it.)

BUG=305317

Review URL: https://codereview.chromium.org/75233002

git-svn-id: svn://svn.chromium.org/blink/trunk@162202 bbb929c8-8fbe-4397-9dbb-9b2b20218538