Use the main world script context for mime handler postMessage.

Use the main world script context for mime handler postMessage.

Previously, calling postMessage from an isolated world would use that
world's context for calling the proxy frame's postMessage(). This
resulted in "Illegal invocation" errors. This CL avoids this problem by
using the main world script context when calling postMessage on the
proxy frame.

BUG=453184
Committed: https://crrev.com/438db6aecb5242715010c129ea508a80c16f9fa9
Cr-Commit-Position: refs/heads/master@{#313665}

[Sam McNally, 2015-01-29 03:00:22]

Patchset #1 (id:1) has been deleted

[Sam McNally, 2015-01-29 03:01:10]

sammc@chromium.org changed reviewers:
+ raymes@chromium.org

[raymes, 2015-01-29 03:25:34]

lgtm

[Sam McNally, 2015-01-29 03:26:42]

The CQ bit was checked by sammc@chromium.org

[commit-bot: I haz the power, 2015-01-29 03:27:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/861073004/20001

[commit-bot: I haz the power, 2015-01-29 04:17:37]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-01-29 04:18:21]

Patchset 1 (id:??) landed as
https://crrev.com/438db6aecb5242715010c129ea508a80c16f9fa9
Cr-Commit-Position: refs/heads/master@{#313665}