Make the destructor of WebFrameClient virtual.

Most subclasses of WebFrameClient assume that the destructor of WebFrameClient
is virtual. Currently, CreateChildCounterFrameClient class in WebViewTest.cpp
is in trouble because of this.

BUG=N/A

Review URL: https://codereview.chromium.org/320583002

git-svn-id: svn://svn.chromium.org/blink/trunk@175685 bbb929c8-8fbe-4397-9dbb-9b2b20218538