BrowserPlugin: Simplify guest access

BrowserPlugin: Simplify guest access

Now that it's impossible for content to create guests and it's impossible
for a compromised embedder to access a guest it does not have permission to
access from content, BrowserPluginGuestManager::MaybeGetGuestByInstanceIDOrKill
seems a bit heavy-handed. This CL simplifies the API a bit.

This is also a prerequiste to support accessibility in BrowserPlugin.

BUG=226145
Committed: https://crrev.com/986f59179731e7970d99d9e8d1a7de43719c73bc
Cr-Commit-Position: refs/heads/master@{#292060}

[Fady Samuel, 2014-08-26 22:41:24]

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

[Fady Samuel, 2014-08-26 22:41:24]

H Charlie,

This is the change we talked about offline in email as a prerequisite for
accessibility work.

Thanks,

Fady

[Charlie Reis, 2014-08-26 23:50:36]

Thanks!  LGTM.

[Fady Samuel, 2014-08-26 23:56:52]

The CQ bit was checked by fsamuel@chromium.org

[commit-bot: I haz the power, 2014-08-26 23:58:47]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/fsamuel@chromium.org/511643002/1

[commit-bot: I haz the power, 2014-08-27 01:11:48]

Committed patchset #1 (1) as c026f9fd691678ed87b91312f57545b2bc811468

[commit-bot: I haz the power, 2014-09-10 02:48:26]

Patchset 1 (id:??) landed as
https://crrev.com/986f59179731e7970d99d9e8d1a7de43719c73bc
Cr-Commit-Position: refs/heads/master@{#292060}