Add include_examples GYP variable.

To make it possible to exclude the examples when running
GYP on all.gyp.
The webrtc_examples.gyp already has an OS=="android" condition
inside it, so there's no need to check that before including it.

BUG=webrtc:4242

Review URL: https://codereview.webrtc.org/1196623006

Cr-Commit-Position: refs/heads/master@{#9556}
1 file changed