Avoid ambiguous overload for operator= in FrameView.h

Followup to CL 459633002, this fixes a compile error with some
older toolchains.

BUG=401254

Review URL: https://codereview.chromium.org/558283003

git-svn-id: svn://svn.chromium.org/blink/trunk@181772 bbb929c8-8fbe-4397-9dbb-9b2b20218538