Merge WebKit r153448 (Optimizes the number of updateTextTrackDisplay calls)

Calling updateTextTrackDisplay() will cause the entire DOM subtree containing the active cues
to be torn down and rebuilt. Only call into this method when text tracks have actually changed.

Add an enum parameter to configureTextTrackDisplay() which allows the caller to specify that
method should assume that the list of visible text tracks has changed, forcing a call to
updateTextTrackDisplay().

http://trac.webkit.org/changeset/153448

R=acolwell
TEST=Existing tests pass.

Review URL: https://chromiumcodereview.appspot.com/22645014

git-svn-id: svn://svn.chromium.org/blink/trunk@158530 bbb929c8-8fbe-4397-9dbb-9b2b20218538