GitLab

Add type annotations to gallery/js/mosaic_mode.js.

BUG=433728
TEST=GYP_GENERATORS=ninja tools/gyp/gyp --depth . ui/file_manager/gallery/js/compiled_resources.gyp && ninja -C out/Default | grep "gallery/js/mosaic_mode.js"

Review URL: https://codereview.chromium.org/805723003

Cr-Commit-Position: refs/heads/master@{#308320}