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The logic in ErrorPageHelper is only applicable when the error
page content is provided by weblayer. If the embedder provides
html for an error page, then ErrorPageHelper shouldn't be used.
This adds a mojom interface to disable ErrorPageHelper for the
next load. I had wanted to delete the instance, but this is
complicated if the frame is reused and would result in having to
duplicate some of the logic.

BUG=1031060
TEST=none

Change-Id: I21d9390a3bfd9dc32084bb77b7a4c1b55c770324
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2330936
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794285}