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This CL updates chrome/chrome_resources.gyp to generate the
chrome_300_percent.pak file (via the newly-added
chrome/chrome_repack_chrome_300_percent.gypi).

Several .grd files have been updated to generate the
*_300_percent.pak files that are used to create
chrome_300_percent.pak.

This CL doesn't add any assets but grit complains if the
necessary default_300_percent directories are not present
so this CL adds placeholder files in these directories until
real assets are added (which is tracked by crbug.com/423755).

Since there are no assets in the *_300_percent directories,
grit will fallback to using the 1x versions when generating the
*_300_percent.pak files.

Note that 300 percent support will be enabled in
ui/base/resources by a subsequent CL
(https://chromiumcodereview.appspot.com/646773004/).

BUG=423762

Review URL: https://codereview.chromium.org/633373010

Cr-Commit-Position: refs/heads/master@{#300869}