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* The current translation screenshot presubmit check flags messages
  which got modified but for which no new translation screenshot
  got uploaded. It thereby also flags messages that got modified, but of
  which the message content itself did not change. This is the case, for
  example, if only the message description for translators got changed.
* This CL restricts the translation screenshot presubmit flagging
  modified messages. It will trigger 1) if the message did not have a
  screenshot at all so far, or 2) if the message content itself changed.
  Only changing the non-visible part of a message that already has a
  translation screenshot does not trigger it anymore.

Bug: 1103844
Change-Id: I254ce2a7267a7d2308b606b16d080c74188d582b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2328669
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797648}