Clarify that a document().frame() access in HTMLMediaElement cannot be null

Access to document().frame() in HTMLMediaElement is always null-checked,
so use the existing frame variable to make it obvious here as well.

R=fs@opera.com

Review URL: https://codereview.chromium.org/1411883003

Cr-Commit-Position: refs/heads/master@{#355771}
1 file changed