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Currently ArcDownloadsWatcherService requests Android media provider to
scan every new/modified files under the Downloads directory. It causes
heavy load when many files are created/deleted there.

This change excludes files not supported by Android media scanner.

BUG=chromium:651731
TEST=touch a.txt  # Not scanned
TEST=touch a.png  # Scanned
TEST=unit_tests --gtest_filter='ArcDownloadsWatcherServiceTest.*'

Review-Url: https://codereview.chromium.org/2379323002
Cr-Commit-Position: refs/heads/master@{#422723}