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-Commit-Position: refs/heads/master@{#348802}

build/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,

ui/ozone/ozone.gni

@@ -26,6 +26,7 @@ declare_args() {
     # Build all platforms whose deps are in install-build-deps.sh.
     # Only these platforms will be compile tested by buildbots.
     ozone_platform_drm = true
+    ozone_platform_gbm = true
     ozone_platform_test = true
     ozone_platform_egltest = true
   }