[dawn] small fix for BUILD.gn

BackendBinding.h includes dawn/dawncpp.h, so dawn_headers_gen needs to be in public_deps.

See below reference for more details.
https://gn.googlesource.com/gn/+/master/docs/reference.md#var_public_deps

Applied `gn format` too.

Bug: chromium:931596
Change-Id: I8572ba54b56c96950a58ec068ba797b3031f3c01
Reviewed-on: https://dawn-review.googlesource.com/c/4640
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index bdd9141..d51eb5c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -12,9 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import("scripts/dawn_overrides_with_defaults.gni")
-import("scripts/dawn_features.gni")
 import("//build_overrides/build.gni")
+import("scripts/dawn_features.gni")
+import("scripts/dawn_overrides_with_defaults.gni")
 
 import("//testing/test.gni")
 
@@ -849,6 +849,11 @@
     "src/utils/TerribleCommandBuffer.cpp",
     "src/utils/TerribleCommandBuffer.h",
   ]
+
+  public_deps = [
+    ":dawn_headers_gen",
+  ]
+
   deps = [
     ":dawn_common",
     ":libdawn_native",