<webview>: Add a screenshot capture API.

Introduce webview.captureVisibleRegion() to get a snapshot of the currently visible
content in the guest-page. This is modelled around the tabs.captureVisibleTab() api.

In order to share code between the two implementation, the core of the capture code
is moved into CaptureWebContentsFunction, with appropriate interface so that this is
usable for both tabs and webview implementation. This refactoring is modelled around
the refactor work that was done for sharing ExecuteScript() implementation between
the two api.

BUG=326755
R=fsamuel@chromium.org, mpcomplete@chromium.org

Review URL: https://codereview.chromium.org/103603003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239642 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239642