Fold localizedMediaControlElementString() and localizedMediaControlElementHelpText() into AccessibilityMediaControls.cpp.

It's ok to use WebKit::Platform directly in core/accessibility/.

Also, these functions took a string argument which identified media controls. It
had no benefit in Blink and we had better use "switch (controlType())" in
accessibilityDescription() and helpText().

BUG=298020
TEST=none; no behavior changes

Review URL: https://codereview.chromium.org/26229002

git-svn-id: svn://svn.chromium.org/blink/trunk@159074 bbb929c8-8fbe-4397-9dbb-9b2b20218538