<webview>: Define API endpoints for webview.contextMenus API.

We expose the following methods in experimental shim:
<webview>.contextMenus.create
<webview>.contextMenus.update
<webview>.contextMenus.remove
<webview>.contextMenus.removeAll

custom_bindings for webview parses the parameters to the function and
passes it to webview_api.cc, which currently does nothing.

Note that I've not included the changes to expose per <webview> event handler
for click (<webview>.contextMenus.onClicked.addListener/removeListener...).
This is because it would further complicate the CL and I don't have proper way to deal /w some parts of it yet.
Per menu-item click is only exposed.

Quick snippet/doc here: https://docs.google.com/a/chromium.org/document/d/1AoMD6kF8ui1dikzTrwK-weVHegSqQaLV2zx4xJSh_fQ/edit

BUG=None
Test=None

Review URL: https://codereview.chromium.org/181013002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254462 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254462