[ MimeHandlerView ] Clean-up unused declarations

The attaching logic and FrameNavigationHelper are now moved to another class
(MimeHandlerViewAttachHelper). This CL removes the left over code.

TBR=wjmaclean@chromium.org

Bug: 659750
Change-Id: Ib67d21ab8104e957bf41df0d20ddf99a04dac3d3
Reviewed-on: https://chromium-review.googlesource.com/c/1460596
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630344}
2 files changed