Revert of Add support for ContentViewCore helpers on Tab

Revert of Add support for ContentViewCore helpers on Tab (patchset #7 id:120001 of https://chromiumcodereview.appspot.com/900443003/)

Reason for revert:
Downstream CL didn't land cleanly.  Need to revert.

Original issue's description:
> Add suppor for ContentViewCore helpers on Tab
>
> - Add a concept of a helper ContentViewCore to Tab.
> - Properly attach these ContentViewCores to the Tabs rendering layer.
> - Note that these aren't rendered by ContentViewRenderView yet.  We need a Chrome-level
> rendering object for that.
>
> BUG=
>
> Committed: https://crrev.com/307787f6e1fd7f3affa872faf6e2efefcc51eb73
> Cr-Commit-Position: refs/heads/master@{#315405}

TBR=dfalcantara@chromium.org,changwan@chromium.org,jaekyun@chromium.org,tedchoc@chromium.org,enne@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Committed: https://crrev.com/7ad18bfea595fac493788fcb2427c68852b7437d
Cr-Commit-Position: refs/heads/master@{#315467}

[David Trainor- moved to gerrit, 2015-02-10 01:37:03]

Created Revert of Add support for ContentViewCore helpers on Tab

[commit-bot: I haz the power, 2015-02-10 01:37:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/908093003/1

[commit-bot: I haz the power, 2015-02-10 01:38:32]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-10 01:40:35]

Patchset 1 (id:??) landed as
https://crrev.com/7ad18bfea595fac493788fcb2427c68852b7437d
Cr-Commit-Position: refs/heads/master@{#315467}