Revert of PPAPI: Fix GetBrowserInterface race conditions

Revert of PPAPI: Fix GetBrowserInterface race conditions (patchset #6 id:100001 of https://codereview.chromium.org/568793002/)

Reason for revert:
Sorry to revert but I randomly noticed this to be causing a top crasher e.g. go/crash/75090c1e31f33cfd

The reason is because the patch explicitly calls PluginGlobals::SetPluginProxyDelegate with NULL from PpapiThread::Shutdown but the pointer is used immediately inside the function.

I guess it should be a simple fix. I was going to put up a fix which just reset the browser_sender_ to NULL in that case, but then I noticed there were cases where we call GetBrowserSender() and use it without checking whether it is NULL (e.g. https://code.google.com/p/chromium/codesearch#search/&q=%22PluginGlobals::Get()-%3EGetBrowserSender()-%3ESend%22&sq=package:chromium&type=cs)

I felt it would be better to just revert for now, sorry :(

Original issue's description:
> PPAPI: Fix GetBrowserInterface race conditions
>
> BUG=413513
>
> Committed: https://crrev.com/d1b2c8f719b0ab471a476bf53911a3657bb4c06a
> Cr-Commit-Position: refs/heads/master@{#294715}

TBR=teravest@chromium.org,piman@chromium.org,dmichael@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=413513
Committed: https://crrev.com/1d38f6f04696744d6b0e9972e5c95d25485da27e
Cr-Commit-Position: refs/heads/master@{#294782}

[raymes, 2014-09-15 03:45:07]

Created Revert of PPAPI: Fix GetBrowserInterface race conditions

[commit-bot: I haz the power, 2014-09-15 03:45:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/566243004/1

[commit-bot: I haz the power, 2014-09-15 03:48:58]

Committed patchset #1 (id:1) as d2abba86be4251e2c79ce58b280c4dd6034e0aea

[commit-bot: I haz the power, 2014-09-15 03:59:57]

Patchset 1 (id:??) landed as
https://crrev.com/1d38f6f04696744d6b0e9972e5c95d25485da27e
Cr-Commit-Position: refs/heads/master@{#294782}