mac: Suppress a clang warning in CustomFrameView.
Clang's static analyzer emits the warning "-Wobjc-designated-initializers" for
CustomFrameView, because CustomFrameView overrides two designated initializers,
but the methods don't call super. The methods throw exceptions, which Clang's
static analyzer isn't smart enough to catch. Declaring the methods with
NS_UNAVAILABLE suppresses the clang warning.
BUG=
Review URL: https://codereview.chromium.org/
1101923002
Cr-Commit-Position: refs/heads/master@{#329971}