gpu/command_buffer/service: Add missing dependency on dawn_platform.

This fixes the roll of Dawn into Chromium after the addition of .cpp
files to the dawn_platform target.

TBR=enga@chromium.org

Bug: None
Change-Id: Ia1d34e7f5eaee615dda9138524a516fd35e80535
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517525Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jiawei Shao <jiawei.shao@intel.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823956}

gpu/command_buffer/service/BUILD.gn

@@ -390,7 +390,10 @@ target(link_target_type, "gles2_sources") {
   }
 
   if (use_dawn) {
-    deps += [ "//third_party/dawn/src/dawn_native" ]
+    deps += [
+      "//third_party/dawn/src/dawn_native",
+      "//third_party/dawn/src/dawn_platform",
+    ]
   }
 
   if (is_mac) {