Revert of Fix WebViewPlugin::scheduleAnimation crash (patchset #2 id:20001 of https...

Revert of Fix WebViewPlugin::scheduleAnimation crash (patchset #2 id:20001 of https://codereview.chromium.org/1137663006/)

Reason for revert:
This is converting the original crash to another crash. Will need another null check.

Original issue's description:
> Fix WebViewPlugin::scheduleAnimation crash
>
> The crash was probably due to accessing a dangling pointer to the plugin
> container during a small time frame between PepperWebPluginImpl::destroy()
> and the destructor being called. (Speculated from source since no reliable
> repro is found.)
>
> This CL clears eveything in the destroy() function as if the destructor has
> been called, only delaying memory release.
>
> R=tommycli
> BUG=483068
>
> Committed: https://crrev.com/6a9b5b10ff3515adb47718fc23ebe039a9e3b9ff
> Cr-Commit-Position: refs/heads/master@{#329309}

TBR=tommycli@chromium.org,raymes@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=487607,483068

Review URL: https://codereview.chromium.org/1127293004

Cr-Commit-Position: refs/heads/master@{#329715}