Check ozone_platform_gbm before doing gbm stuff.

Just checking use_ozone fails when building an ozone platform which
doesn't have gbm, e.g. chromecast.

BUG=angleproject:1439
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_gn_chromeos_rel;tryserver.chromium.android:android_compile_dbg

Change-Id: Idec14441ce30dd872b3bfddfe844abfeb910cc02
Reviewed-on: https://chromium-review.googlesource.com/358860
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index ac1960c..0478a03 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -7,8 +7,9 @@
 import("//build/config/linux/pkg_config.gni")
 import("//build/config/ui.gni")
 import("//third_party/angle/build/angle_common.gni")
+import("//ui/ozone/ozone.gni")
 
-if (use_ozone) {
+if (ozone_platform_gbm) {
   pkg_config("libdrm") {
     packages = [ "libdrm" ]
   }
@@ -374,7 +375,7 @@
         "log",
       ]
     }
-    if (use_ozone) {
+    if (ozone_platform_gbm) {
       configs += [ ":libdrm" ]
       defines += [ "ANGLE_USE_OZONE" ]
       deps += [ "//third_party/minigbm" ]