<webview>: Fix text selection features in mac.
We still need to pass RWHVGuest::SelectionChanged() call to platform_view_.
This broke while adding IME support, this function doesn't seem to be required for IME (made sure by running BrowserPluginHostTest.InputMethod).

Wrote a browser_test to catch the regression.

BUG=331722
Test=Open a <webview> in a chrome app. Select some text. Observe that "Look up in dictionary" and "Speech=>Start Speaking" features work.

Review URL: https://codereview.chromium.org/148433007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247589 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247589