<webview>: Enable Drag and Drop on all platforms.

The known issues for <webview> dnd have been resolved.
This CL just enables drag/drop, more cleanup will follow to remove
content flags.

BUG=341230
Test=Drag and drop within <webview> will work on all platforms, instead
of just chromeos.

Review URL: https://codereview.chromium.org/173713005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252472 0039d316-1c4b-4281-b951-d872f2087c98
ui: Make layer_unittest own a CompositorHost instead of a Compositor.

This means the test will have a window so it can create a view
context.

TBR=piman@chromium.org
BUG=270918

Review URL: https://codereview.chromium.org/170473004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252473 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252473