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More accurately, this just hides the codepath that would redirect files
sniffed as "image/tiff" back to the old gallery app if the file had an
extension matching a known RAW image format. (chrome://media-app was
already a handler for "image/*", so logic was needed to exclude RAW).

However, it also adds explicit extension-match file handlers to the
web app manifest to handle cases where sniffing is not done. Initially
tried sharing the logic that drives `IsRawFile()` in the files app's
file_tasks but that's not really compatible with the web app manifest,
and `IsRawFile` can just be deleted soon. Encapsulate IsRawFile() to
make that removal easier: nothing outside file_tasks.cc uses it since
r777758.

The logic is put behind a base::Feature, but not a chrome://flag: usage
of this is low and the change is low-risk. And flipping
chrome://flags#media-app also works to use the old gallery for these
file types.

Bug: 1030935, b/154062029
Change-Id: I4779f76fa81800ffef316748cb94e12534b38775
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494404Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820645}