When screen/window capturers fail to use shared memory, they are
supposed to fall back to XGetImage(). It's slower, but should still
allow to capture desktop content. This wasn't implemented correclty in
XScreenPixelBuffer - it was failing completely when shmget() returns an
error.
BUG=705146
Review-Url: https://codereview.webrtc.org/2775793003
Cr-Commit-Position: refs/heads/master@{#17401}