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Currently, ArcDownloadsWatcher is used to watch only Downloads directory
in ChromeOS and triggers MediaScanner.scanFile when important file
changes events happen (for example, file creation, deletion, etc).

In exact same way, we want to watch changes under removable media. This
CL generalize ArcDownloadsWatcher to watch both paths.

BUG=b:69078137
TEST=existing unit test and manually observe logs during runtime

Change-Id: I9adce1964eb6b23c2e83ead8462b7a1d8985ae4a
Reviewed-on: https://chromium-review.googlesource.com/c/1288160
Commit-Queue: Risan <risan@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601802}