[GuestView] - Introduce MimeHandlerViewAttachHelper

This CL is for the most part a mechanical change which extracts almost
all the frame-based MimeHandlerView code out of
ExtensionsGuestViewMessageFilter. This change both removes the current
clutter form EGVMF as well as fixesa race introduced when the
frame-based logic was added to EGVMF. The reason for the race was that
EGVMF is destroyed on IO thread but all the access to it (for
frame-based MHV) are from UI.

TBR=avi@chromium.org,lazyboy@chromium.org

Bug: 659750, 896679, 911161, 918861
Change-Id: I6474b870e4d56daa68be03637bb633665d9f9dda
Reviewed-on: https://chromium-review.googlesource.com/c/1401451
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621155}
void ExtensionsGuestViewMessageFilter::FrameNavigationHelper::FrameDeleted(

void ExtensionsGuestViewMessageFilter::FrameNavigationHelper::FrameDeleted(
    RenderFrameHost* render_frame_host) {
  if (render_frame_host->GetFrameTreeNodeId() != frame_tree_node_id_)
    return;
  filter_->ResumeAttachOrDestroy(element_instance_id_,
                                 MSG_ROUTING_NONE /* no plugin frame */);
}
