Add null-checks before dereferencing.

We got a number of crash reports with null frameView in contentsSize().
The surrounding code also suggests that these checks might be necessary.

BUG=

Review URL: https://codereview.chromium.org/538013002

git-svn-id: svn://svn.chromium.org/blink/trunk@182106 bbb929c8-8fbe-4397-9dbb-9b2b20218538