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

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

Committed: https://crrev.com/f2c7f598dd277d9b79799e713da669bf29b629b9
Cr-Commit-Position: refs/heads/master@{#355771}

[philipj_slow, 2015-10-23 11:22:07]

The CQ bit was checked by philipj@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2015-10-23 11:22:16]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1411883003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1411883003/1

[fs, 2015-10-23 11:25:14]

lgtm

[commit-bot: I haz the power, 2015-10-23 12:33:16]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-23 12:33:17]

Dry run: This issue passed the CQ dry run.

[philipj_slow, 2015-10-23 13:15:56]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2015-10-23 13:16:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1411883003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1411883003/1

[commit-bot: I haz the power, 2015-10-23 13:19:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-23 13:21:07]

Patchset 1 (id:??) landed as
https://crrev.com/f2c7f598dd277d9b79799e713da669bf29b629b9
Cr-Commit-Position: refs/heads/master@{#355771}