Revert of Remove Platform::current checks from Mojo's error handlers in geolocation (patchset #1 id…

Revert of Remove Platform::current checks from Mojo's error handlers in geolocation (patchset #1 id:1 of https://codereview.chromium.org/2271953002/ )

Reason for revert:
I'll revert r413430 and its dependent CLs because r413430 caused issue 642072.

The problem is that we cannot simply remove blink::shutdown because the following scenario can happen:

1) blink::shutdown is not called. Workers are still running.
2) RenderThreadImpl gets destructed. MessageLoop gets destructed.
3) The workers may access the RenderThreadImpl and MessageLoop.

To fix the problem, we need to call ProcessDied() and forcibly kill the renderer process at the end of RenderThreadImpl::Shutdown().

Original issue's description:
> Remove Platform::current checks from Mojo's error handlers in geolocation
>
> The checks were added in https://codereview.chromium.org/2163683004/
> but are no longer needed because I already removed blink::shutdown
> from the renderer's shutdown sequence.
>
> BUG=628999, 639244
>
> Committed: https://crrev.com/8435863a81be1eee4c28131a341f8a4bce04a7b4
> Cr-Commit-Position: refs/heads/master@{#414232}

TBR=sammc@chromium.org
BUG=628999, 639244
NOTRY=true

Review-Url: https://codereview.chromium.org/2312553003
Cr-Commit-Position: refs/heads/master@{#416480}
(cherry picked from commit 556b4223eeea55a338bf38ca87b004ba23e1039a)

[haraken, 2016-09-06 05:14:52]

Committed patchset #1 (id:1) to pending queue manually as
64308ecc3480d4f9c3882b37a7f9689cb4c1bcde.