Ozone: Only set angle_enable_gl=1 for the DRM/GBM platform.

Currently, the load of DRM dependencies and definition of ANGLE_USE_OZONE
are conditioned by ozone_platform_gbm in the GN build files. However,
angle_enable_gl is always turned on for use_ozone=1. As a consequence,
use_ozone=1 and ozone_platform_gbm=0 (default on Linux Desktop) produces
the following error:

    ../../third_party/angle/src/libANGLE/Display.cpp:52:9: error: Unsupported OpenGL platform.
    #error Unsupported OpenGL platform.

    ../../third_party/angle/src/libANGLE/Display.cpp:192:2: error: Unsupported OpenGL platform.
    #error Unsupported OpenGL platform.

This CL turns off angle_enable_gl in that case to fix that build error.

Change-Id: I42578f30f3353c083f52d4bd345dbe33f3f5e12e
Reviewed-on: https://chromium-review.googlesource.com/408100
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>

gni/angle.gni

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")  # import the use_x11 variable
+import("//ui/ozone/ozone.gni")
 
 angle_enable_d3d9 = false
 angle_enable_d3d11 = false
@@ -19,7 +20,7 @@ if (is_win) {
   import("//build/config/win/visual_studio_version.gni")
 } else if (is_linux && use_x11 && !is_chromeos) {
   angle_enable_gl = true
-} else if (is_mac || use_ozone) {
+} else if (is_mac || ozone_platform_gbm) {
   angle_enable_gl = true
 } else if (is_android) {
   angle_enable_gl = true