Code cleanup: remove unnecessary casts.

With the three sides of the patch needed for 343327 landed, these temporary
casts are unnecessary. Removed.

BUG=343327

Review URL: https://codereview.chromium.org/313263005

git-svn-id: svn://svn.chromium.org/blink/trunk@175797 bbb929c8-8fbe-4397-9dbb-9b2b20218538