Fix shared libary build for content_browsertests.

Move the WebMIDIClientMock destructor implementation from
the header to the cpp. This way we can avoid undefined
vtable errors.

BUG=415162

Review URL: https://codereview.chromium.org/574283004

git-svn-id: svn://svn.chromium.org/blink/trunk@182326 bbb929c8-8fbe-4397-9dbb-9b2b20218538