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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.

In additional to the original CL https://codereview.chromium.org/1137663006/
This CL also switched the destruction order of pepper instance and the
throttler. Engaging throttle during destruction cause crash and doesn't make
sense anyway.

R=tommycli,raymes
BUG=483068,487607

Review URL: https://codereview.chromium.org/1141793002

Cr-Commit-Position: refs/heads/master@{#330638}