Revert of Allow new zoom bubble to override current one.

Revert of Allow new zoom bubble to override current one. (patchset #6 id:100001 of https://codereview.chromium.org/849693006/)

Reason for revert:
The changes in this CL are causing a crash: crbug.com/453564

Original issue's description:
> Allow new zoom bubble to override current one.
>
> At present, if a user changes zoom on OSX while the zoom bubble is
> already being shown, the new zoom bubble (with the most up to date
> information) is not shown. This CL changes this behaviour by allowing
> a new zoom bubble to be created, and replace the existing one.
>
> Example: load a PDF and repeatedly click the zoom +/- controls in the
> lower-right corner of the viewer.
>
> BUG=444995
>
> Committed: https://crrev.com/a95ea7031e472fd5d0968955989c963381dfaf5f
> Cr-Commit-Position: refs/heads/master@{#313564}

TBR=rohitrao@chromium.org,erikchen@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=444995
Committed: https://crrev.com/97b62a99d561261001b8e4ddd2a5946a6fa743bb
Cr-Commit-Position: refs/heads/master@{#313785}

[wjmaclean, 2015-01-29 20:55:05]

Created Revert of Allow new zoom bubble to override current one.

[commit-bot: I haz the power, 2015-01-29 20:56:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/867183006/1

[commit-bot: I haz the power, 2015-01-29 20:57:18]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-29 20:59:33]

Patchset 1 (id:??) landed as
https://crrev.com/97b62a99d561261001b8e4ddd2a5946a6fa743bb
Cr-Commit-Position: refs/heads/master@{#313785}