Use the WebTestProxy for layout tests in content_shell

- Add a hook to the content renderer client to allow embedders to override the
  creation of RenderViewImpls. To simplify this, I consolidate the parameters
  passed to RVI's constructor into a struct.

- Add a content_layouttest_support library that provides a method for creating
  a modified RenderViewImpl suitable for running layout tests.

- Use above library from the content_shell.

BUG=111316
TEST=almost all accessibility tests pass.

Review URL: https://codereview.chromium.org/11362161

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@167341 0039d316-1c4b-4281-b951-d872f2087c98
14 files changed