ScreenCapturerMac wasn't handling the following two cases properly
which could cause crashes:
1. CGDisplayCreateImage() returns image with depth other than 32-bit
2. CGDisplayCreateImage() returns image with dimensions different
from expected (e.g. when screen resolution is being changed).
I suspect that (2) was causing the linked bug.
BUG=crbug.com/504927
R=jiayl@webrtc.org
Review URL: https://codereview.webrtc.org/1816723002 .
Cr-Commit-Position: refs/heads/master@{#12077}