[4/N] Add DIR_METADATA files to directories under //content

Generate DIR_METADATA files and remove metadata from OWNERS
files for directories under //content.

R=boliu@chromium.org, dtseng@chromium.org, kbr@chromium.org

Bug: 1113033
Change-Id: Id2c963398829473e034373d3dc7909d325c77c0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2543695
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828501}