Fixes for build in Chromium

- Fix missing include in span.h which breaks on libc++ modules builds.
- Fix missing suppressions for -Wsign-compare which is enabled by
  -Wextra in the default `chromium_code` config.
- Remove gtest from --system-header-prefix in Dawn standalone because
  it's no longer needed.

Roll failure here: https://crrev.com/c/8091350

Bug: none, 523358231
Change-Id: I727819e7997c915284d820deb248c1d55db1eddb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/324796
Reviewed-by: Shrek Shao <shrekshao@google.com>
Auto-Submit: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/utils/BUILD.gn

diff --git a/src/utils/BUILD.gn b/src/utils/BUILD.gn
index fd6be6f..1b6860e 100644
--- a/src/utils/BUILD.gn
+++ b/src/utils/BUILD.gn
@@ -149,7 +149,6 @@
           "benchmark/",
           "glslang/",
           "gmock/",
-          "gtest/",
           "KHR/",
           "partition_alloc/",
           "napi.h",

src/utils/span.h

diff --git a/src/utils/span.h b/src/utils/span.h
index 3dcb1bd..55de9df 100644
--- a/src/utils/span.h
+++ b/src/utils/span.h
@@ -31,6 +31,7 @@
 #include <algorithm>
 #include <concepts>
 #include <cstdint>
+#include <functional>
 #include <limits>
 #include <memory>
 #include <new>

src/utils/warning_suppression_mappings.txt

diff --git a/src/utils/warning_suppression_mappings.txt b/src/utils/warning_suppression_mappings.txt
index 335b096..39e6a63 100644
--- a/src/utils/warning_suppression_mappings.txt
+++ b/src/utils/warning_suppression_mappings.txt
@@ -1,3 +1,7 @@
+# Per-file per-warning suppressions.
+# Every suppression should have a brief explanation of why it's needed.
+# Note: make sure there is only one section per warning - later sections overwrite earlier ones.
+
 [language-extension-token]
 # For MSVC-specific extensions: __FUNCSIG__, __uuidof, IID_PPV_ARGS.
 src:*src/dawn/native/Surface.cpp
@@ -46,3 +50,11 @@
 [double-promotion]
 # Performance warning for float-to-double promotion, common in tests.
 src:*src/dawn/tests/*
+# Also happens inside GTest macros like EXPECT_EQ.
+# Path differs currently in Dawn vs Chromium.
+src:*third_party/googletest/{,src/}googletest/include/gtest/gtest.h
+
+[sign-compare]
+# Happens inside EXPECT_EQ macros and similar in our tests. Should be fixed, but not critical.
+# Path differs currently in Dawn vs Chromium.
+src:*third_party/googletest/{,src/}googletest/include/gtest/gtest.h