[DevTools] Fix potential crash in RenderFrameDevToolsAgentHost

[DevTools] Fix potential crash in RenderFrameDevToolsAgentHost

BUG=483693
Committed: https://crrev.com/1771428fefada2d24fab5970b915b941a5014cc1
Cr-Commit-Position: refs/heads/master@{#330286}

[vkuzkokov, 2015-05-15 19:17:07]

vkuzkokov@chromium.org changed reviewers:
+ dgozman@chromium.org

[vkuzkokov, 2015-05-15 19:17:08]

First crash in crbug.com/483693 is more time-consistent with
https://codereview.chromium.org/1094153002
rather than the CL reported in bug.
The possible culprit is this line:
https://code.google.com/p/chromium/codesearch#chromium/src/content/browser/de...
web_contents() might still be not null when render_frame_host_ is already null.
This patch fixes that.

[pfeldman, 2015-05-15 23:06:37]

pfeldman@chromium.org changed reviewers:
+ pfeldman@chromium.org

[pfeldman, 2015-05-15 23:06:38]

lgtm

[dgozman, 2015-05-16 01:35:50]

Good catch! lgtm

[vkuzkokov, 2015-05-16 11:23:16]

The CQ bit was checked by vkuzkokov@chromium.org

[commit-bot: I haz the power, 2015-05-16 11:23:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1132913004/1

[commit-bot: I haz the power, 2015-05-16 11:26:11]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-18 11:31:50]

Patchset 1 (id:??) landed as
https://crrev.com/1771428fefada2d24fab5970b915b941a5014cc1
Cr-Commit-Position: refs/heads/master@{#330286}

[vkuzkokov, 2015-05-18 11:39:52]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1140603004/ by vkuzkokov@chromium.org.

The reason for reverting is: See crbug.com/489135.