Zoom Extension API (content changes)

Changes to content/ needed to facilitate the new zoom extension API,
which is outlined in:

https://docs.google.com/a/chromium.org/document/d/1sCZjx1J3_M2a02T8NXd-ufGKZDoBHI5Ixis1DaGLQCA/edit?usp=sharing

Based on code from https://codereview.chromium.org/226523006/, with
various fixes and rebased against changes in
https://codereview.chromium.org/287093002/

Must land before https://codereview.chromium.org/301733006/.

BUG=30583

Review URL: https://codereview.chromium.org/302603012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@277153 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed