Initialize render_frame_message_source_ in WebContentsImpl ctor.

Initialize render_frame_message_source_ in WebContentsImpl ctor.

Yet another cluster-fuzz crash happens due to invalid
render_frame_message_source_. As I see the code, this field is
always reset to NULL at the every IPC message but not initialized
at all, therefore the crash may still happen if the message is
the very first message to the object.

BUG=451755
R=jam@chromium.org

Committed: https://crrev.com/1db093769786287b1a34ef49c30ecf6bf64a7bfc
Cr-Commit-Position: refs/heads/master@{#313335}

[Charlie Reis, 2015-01-26 01:03:21]

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

[Charlie Reis, 2015-01-26 01:03:22]

LGTM.

[Jun Mukai, 2015-01-26 06:48:58]

The CQ bit was checked by mukai@chromium.org

[commit-bot: I haz the power, 2015-01-26 06:49:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/869053004/1

[commit-bot: I haz the power, 2015-01-26 07:52:13]

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

[commit-bot: I haz the power, 2015-01-26 07:52:14]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Jun Mukai, 2015-01-26 18:07:37]

The CQ bit was checked by mukai@chromium.org

[commit-bot: I haz the power, 2015-01-26 18:08:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/869053004/1

[commit-bot: I haz the power, 2015-01-26 18:08:21]

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

[commit-bot: I haz the power, 2015-01-26 18:08:22]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Jun Mukai, 2015-01-27 18:31:06]

The CQ bit was checked by mukai@chromium.org

[commit-bot: I haz the power, 2015-01-27 18:32:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/869053004/1

[commit-bot: I haz the power, 2015-01-27 19:05:15]

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

[commit-bot: I haz the power, 2015-01-27 19:05:16]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[inferno, 2015-01-27 19:37:18]

The CQ bit was checked by inferno@chromium.org

[commit-bot: I haz the power, 2015-01-27 19:38:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/869053004/1

[commit-bot: I haz the power, 2015-01-27 20:07:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-27 20:09:28]

Patchset 1 (id:??) landed as
https://crrev.com/1db093769786287b1a34ef49c30ecf6bf64a7bfc
Cr-Commit-Position: refs/heads/master@{#313335}