Enable gbm platform by default in ozone builds

Now that we have minigbm in third_party we can set ozone_platform_gbm=1 by default.

BUG=530712
TEST=tryjobs for linux_chromium_chromeos_ozone_rel_ng,
  cast_shell_linux

Review URL: https://codereview.chromium.org/1336023002

Cr-Original-Commit-Position: refs/heads/master@{#348802}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 88e504e383e7641e139bc836b33ded00a0d46668

common.gypi

@@ -2385,6 +2385,7 @@
 
         # Build all platforms whose deps are in install-build-deps.sh.
         # Only these platforms will be compile tested by buildbots.
+        'ozone_platform_gbm%': 1,
         'ozone_platform_drm%': 1,
         'ozone_platform_test%': 1,
         'ozone_platform_egltest%': 1,