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In preparation for using the observer in WebContentsImpl, make it
available for content/.

Bug: 889087
Change-Id: Iff4dbfae5525c6bae1655662d5f29208a024da28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574802Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Ramya Nagarajan <ramyan@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653639}