gpu: Fix build_gles2_cmd_buffer.py clang-format on Windows

Windows expects back slash instead of forward slash in paths. Python
subprocess.call does not convert slashes automatically.

BUG=NONE
R=zmo

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I76d8d4b973127102b42a4a02f59b09993383aabb
Reviewed-on: https://chromium-review.googlesource.com/956548Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541918}

gpu/command_buffer/build_cmd_buffer_lib.py

@@ -6929,6 +6929,6 @@ const size_t %(p)sUtil::enum_to_string_table_len_ =
 def Format(generated_files):
   formatter = "third_party/depot_tools/clang-format"
   if platform.system() == "Windows":
-    formatter += ".bat"
+    formatter = "third_party\\depot_tools\\clang-format.bat"
   for filename in generated_files:
     call([formatter, "-i", "-style=chromium", filename])