This is a trivial but dangerous change to remove X11/Xlib.h out of
shared_x_display.h. Since we do not have a strict and automatically
Include-What-You-Use rule, I cannot quite tell whether any Chromium source files
wrongly assume X11/Xlib.h will be included through shared_x_display.h. We can
fix the breaks in Chromium after this change has been failed to integrate to
Chromium.
BUG=webrtc:6513
Review-Url: https://codereview.webrtc.org/2482963003
Cr-Commit-Position: refs/heads/master@{#14987}
e083909f85