Update Mesa build variables.

For gclient: angle_mesa -> checkout_angle_mesa
For BUILD.gn: build_angle_mesa gets set automatically.

Bug: angleproject:7689
Change-Id: I89ff8a54fc87d9fb28ac998331ffbd0d8eb246ff
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3989923
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>

DEPS

@@ -9,6 +9,7 @@ gclient_gn_args_file = 'build/config/gclient_args.gni'
 
 gclient_gn_args = [
   'checkout_angle_internal',
+  'checkout_angle_mesa',
   'checkout_angle_restricted_traces',
   'generate_location_tags',
 ]
@@ -39,7 +40,7 @@ vars = {
   'checkout_android_native_support': 'checkout_android or checkout_chromeos',
 
   # Check out Mesa and libdrm in ANGLE's third_party folder.
-  'angle_mesa': False,
+  'checkout_angle_mesa': False,
 
   # Version of Chromium our Chromium-based DEPS are mirrored from.
   'chromium_revision': 'c8d10c0b09aba42a2849e283bfea79db014d2ac5',

gni/angle.gni

@@ -215,6 +215,10 @@ if (!angle_enable_cl) {
   angle_enable_cl_passthrough = false
 }
 
+if (!defined(checkout_angle_mesa)) {
+  checkout_angle_mesa = false
+}
+
 declare_args() {
   # Currently SwiftShader's Vulkan front-end doesn't build on Android.
   # Disabled by default on MSVC because of compiler warnings/errors.
@@ -277,8 +281,7 @@ declare_args() {
   # Vulkan.
   angle_enable_overlay = angle_enable_vulkan && angle_debug_layers_enabled
 
-  # Build Mesa as a third party dependency
-  angle_build_mesa = false
+  angle_build_mesa = checkout_angle_mesa && is_linux
 }
 
 declare_args() {