Create MimeHandlerViewFrameContainer only once

Currently embedding a PDF creates a MimeHandlerViewFrameContainer which
then loads an extension that attaches another internal plugin element
to load the PDF plugin. This CL makes sure we do not create a second
MimeHandlerViewFrameContainer for that.

Bug: 659750
Change-Id: Iff71eee6d59b824572628f1ba68651d6be16138c
Reviewed-on: https://chromium-review.googlesource.com/c/1265502
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597284}
2 files changed