android: Migrate old content readback to use async readback (and delegated renderer)

This patch removes the use of a helper context for content readback.  Instead,
we use the async readback API.

This patch is also setting delegated renderer as the default for android.

android= https://chrome-internal-review.googlesource.com/#/c/152377/

BUG=326363
TBR=sievers
NOTRY=true

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=248827

Review URL: https://codereview.chromium.org/143803004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249081 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249081