[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::OverrideThreadForMessage(
    const IPC::Message& message,
    BrowserThread::ID* thread) {
  switch (message.type()) {
    case ExtensionsGuestViewHostMsg_ResizeGuest::ID:
      *thread = BrowserThread::UI;
      break;
    default:
      GuestViewMessageFilter::OverrideThreadForMessage(message, thread);
  }
}

 void ExtensionsGuestViewMessageFilter::OverrideThreadForMessage(
    const IPC::Message& message,
    BrowserThread::ID* thread) {
  switch (message.type()) {
    case ExtensionsGuestViewHostMsg_ResizeGuest::ID:
      *thread = BrowserThread::UI;
      break;
    default:
      GuestViewMessageFilter::OverrideThreadForMessage(message, thread);
  }
}
