The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/070f876a788a7e2a0cb6852bcd4f92d31042c941

commit 070f876a788a7e2a0cb6852bcd4f92d31042c941
Author: Mike Dougherty <michaeldo@chromium.org>
Date: Wed Dec 12 19:53:37 2018

Ensure that WebFramesManager->GetMainWebFrame() always returns a valid WebFrame.

Remove extraneous |removeAllWebFrames| call to ensure a WebFrame exists for the main frame when navigating back then forward on a same page (ex:example.com#link) link.

Additionally, add tests to validate the returned WebFrame object.

Bug:  913825 
Change-Id: I5abe3a50769564c1c95157eb713ee435093cbab4
Reviewed-on: https://chromium-review.googlesource.com/c/1368768
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616004}
[modify] https://crrev.com/070f876a788a7e2a0cb6852bcd4f92d31042c941/ios/web/web_state/ui/crw_web_controller.mm
[modify] https://crrev.com/070f876a788a7e2a0cb6852bcd4f92d31042c941/ios/web/web_state/web_frames_manager_inttest.mm