[iOS][Translate] Do nothing on revertTranslation if script isn't injected

Changes the DCHECK in [JsTranslateManager revertTranslation] to an early
return if the script hasn't been injected. Since there is no API in iOS to
check whether the page has been translated, there is no way to stop the
user from attempting to revert the translation by tapping the source
language button in the new translate infobar.

Bug: 910994
Change-Id: Ie6accd783db2039acdfd95acb968bf8fb52da98b
Reviewed-on: https://chromium-review.googlesource.com/c/1425958
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: anthonyvd <anthonyvd@chromium.org>
Auto-Submit: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: anthonyvd <anthonyvd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624674}
1 file changed