Allow c2y-extensions when using CMake

Applies https://dawn-review.googlesource.com/c/dawn/+/275695 when
building with CMake.

Bug: 452840620
Change-Id: If6f1133a658f3e2eebba9aacbf3a7752fa56f7d1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/276054
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Auto-Submit: Brian Sheedy <bsheedy@google.com>
Commit-Queue: Brian Sheedy <bsheedy@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/cmake/DawnCompilerWarningFlags.cmake

diff --git a/src/cmake/DawnCompilerWarningFlags.cmake b/src/cmake/DawnCompilerWarningFlags.cmake
index 7be0dc1..1aad8b9 100644
--- a/src/cmake/DawnCompilerWarningFlags.cmake
+++ b/src/cmake/DawnCompilerWarningFlags.cmake
@@ -111,6 +111,10 @@
 dawn_add_flag("-Wunused-macros" ${langs})
 
 if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+  # Flags use of __COUNTER__, which is used in histogram macros that were
+  # largely copied from Chromium. Chromium continues to use __COUNTER__ in
+  # their copy.
+  dawn_add_flag("-Wno-c2y-extensions")
   if (CMAKE_CXX_COMPILER_VERSION_MAJOR VERSION_LESS_EQUAL 18)
     dawn_add_flag("-Wno-gcc-compat" ${langs})
   endif ()