Merge 94721 - FrameLoader::addExtraFieldsToRequest can crash when called from or after FrameLoade...

Merge 94721 - FrameLoader::addExtraFieldsToRequest can crash when called from or after FrameLoader::detachFromParent
https://bugs.webkit.org/show_bug.cgi?id=61810

Reviewed by Eric Seidel.

Fix this crash as suggested by Darin Fisher in
https://bugs.webkit.org/show_bug.cgi?id=61810#c21.  This patch does not
include the test requested by Alexey Proskuryakov in
https://bugs.webkit.org/show_bug.cgi?id=61810#c6.  I would really much
rather include a test with this patch, but my attempts to write a test
have failed.  :(

* src/WebPluginContainerImpl.cpp:
(WebKit::WebPluginContainerImpl::loadFrameRequest):


TBR=abarth@webkit.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=94747