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This CL is making const the function that are not modifying the
state of the resource bundle.

This step is required to make the transition to a Reader/Writer
lock.

Bug: 1123115
Change-Id: I764a8657fe358ce8cc5097d7ca7140a48682afd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401324Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805414}