Fix OSX IME, Call RVH::SetInputMethodActive() after Attach.

Fix OSX IME, Call RVH::SetInputMethodActive() after Attach.

This broke quite a bit ago when the implicit ordering changed by refactoring.
It is unfortunate that we don't have test coverage for mac, and
trying to type accent characters (IME) from browsertest didn't work.

BUG=450641
Test=On mac, load a chrome app with <webview> that has an input box.
Try to type accent chars, e.g. Option+e followed by e. Proper chars
should be seen in the input box.

Committed: https://crrev.com/8b0b5d0c0e4be272da4233cfad13f2a688f8eec4
Cr-Commit-Position: refs/heads/master@{#315142}

[lazyboy, 2015-02-06 07:53:51]

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

[Fady Samuel, 2015-02-06 08:13:28]

lgtm

[lazyboy, 2015-02-06 23:03:44]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2015-02-06 23:04:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/904013002/1

[commit-bot: I haz the power, 2015-02-07 00:02:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-07 00:02:51]

Patchset 1 (id:??) landed as
https://crrev.com/8b0b5d0c0e4be272da4233cfad13f2a688f8eec4
Cr-Commit-Position: refs/heads/master@{#315142}