Mojo: Fix Blink-specific export_define_blink directive

Previously, defines in Blink variants would be set to both the
Chromium and the Blink variants export_define* values.  This isn't
correct since Blink variants must export their own defines.

This CL causes Blink variants to only define what's set with
export_define_blink.

Bug: 717812
Change-Id: Ie554cef3044eba6e4ee093098b9c4afedaecd89e
Reviewed-on: https://chromium-review.googlesource.com/c/1282209Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600044}

mojo/public/tools/bindings/mojom.gni

@@ -951,11 +951,15 @@ template("mojom") {
         testonly = invoker.testonly
       }
       defines = []
-      if (defined(invoker.export_define)) {
-        defines += [ invoker.export_define ]
-      }
-      if (defined(invoker.export_define_blink)) {
-        defines += [ invoker.export_define_blink ]
+      if (defined(bindings_configuration.for_blink) &&
+          bindings_configuration.for_blink) {
+        if (defined(invoker.export_define_blink)) {
+          defines += [ invoker.export_define_blink ]
+        }
+      } else {
+        if (defined(invoker.export_define)) {
+          defines += [ invoker.export_define ]
+        }
       }
       if (enabled_sources != []) {
         sources = process_file_template(enabled_sources, generator_cpp_outputs)