Speculative fix for ChromeWebViewGuestDelegate crash.

Speculative fix for ChromeWebViewGuestDelegate crash.

It is possible that when
ChromeWebViewGuestDelegate::HandleContextMenu() attempts to build a
menu to display that it will receive a null pointer (if the associated
web_contents' frame tree does not have a focussed frame). In the
current code this can lead to a crash. In the short term, while we
determine if this lack of a focussed frame is an error or not, early-out
if we receive a null menu.

BUG=487419
Committed: https://crrev.com/5fca2df48962e5812c8dc438c60434f1bcfae6a1
Cr-Commit-Position: refs/heads/master@{#330326}

[wjmaclean, 2015-05-15 01:35:12]

The CQ bit was checked by wjmaclean@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-05-15 01:36:23]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1139193003/1

[wjmaclean, 2015-05-15 01:36:44]

wjmaclean@chromium.org changed reviewers:
+ fsamuel@chromium.org

[wjmaclean, 2015-05-15 01:36:45]

Über-small CL, please take a look.

[commit-bot: I haz the power, 2015-05-15 02:19:43]

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

[commit-bot: I haz the power, 2015-05-15 02:19:44]

Dry run: This issue passed the CQ dry run.

[Fady Samuel, 2015-05-17 09:23:55]

lgtm

[wjmaclean, 2015-05-18 03:17:20]

The CQ bit was checked by wjmaclean@chromium.org

[commit-bot: I haz the power, 2015-05-18 03:17:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1139193003/1

[commit-bot: I haz the power, 2015-05-18 04:10:52]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-18 11:33:32]

Patchset 1 (id:??) landed as
https://crrev.com/5fca2df48962e5812c8dc438c60434f1bcfae6a1
Cr-Commit-Position: refs/heads/master@{#330326}