Convert remaining uses of WebView::initializeMainFrame to setMainFrame.

This is the Blink-side change, which also removes the interfaces, since
Chrome code no longer uses this method. This patch also fixes a memory
leak introduced in the unit tests by r164135 and adds a warning note to
WebView::createChildFrame() to hopefully prevent this from happening
again.

BUG=329266
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/112023012

git-svn-id: svn://svn.chromium.org/blink/trunk@164214 bbb929c8-8fbe-4397-9dbb-9b2b20218538