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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.

R=tommycli
BUG=483068

Review URL: https://codereview.chromium.org/1137663006

Cr-Commit-Position: refs/heads/master@{#329309}