Port the angle_link_glx setting of the gyp build to the standalone gn build.
When set to true, libGL will be linked into libANGLE, instead of dlopen-ed.
This is useful when using apitrace.

BUG=angleproject:1569

Change-Id: I16a91504960169efdcde3b2c50b944de148d9ba6
Reviewed-on: https://chromium-review.googlesource.com/821691Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>