Remove duplicated provideMediaKeysTo() in MediaKeysClient.h

Though MediaKeysController has already provideMediaKeysTo() which is being used by WebViewImpl, MediaKeysClient.h also has provideMediaKeysTo() unnecessary.

BUG=none

Review URL: https://codereview.chromium.org/338713002

git-svn-id: svn://svn.chromium.org/blink/trunk@177184 bbb929c8-8fbe-4397-9dbb-9b2b20218538