Add a new configuration for disabling -Wshorten-64-to-32 warnings.

The old name for this was 'no_size_t_to_int_warning' which was
specific to MSVC. clang does not have a specific warning for just
64-bit size_t -> 32-bit value, but instead it has a warning for
any conversion from 64-bit to 32-bit (-Wshorten-64-to-32).

As part of re-enabling this warning, fixes will be landed to get
Chromium warning-clean with -Wshorten-64-to-32 on 64-bit builds
only (as tackling for 32-bit builds too, which was never covered
by any previous MSVC warning would be too big a first step).

For those projects that are e.g. third party or can be tackled at
a later date, they can include this new config.

The name is different from 'no_size_t_to_int_warning' because that
name is confusing (as the new warning enables more than just this).

New exceptions will be added with 'no_shorten_64_warnings' and
old 'no_size_t_to_int_warning' will be slowly removed or migrated
over time. Then the old config will be removed.

BUG=588506

Change-Id: I3a6ce9b19e52c7bdc724c7fd326f6afe7b509219
Reviewed-on: https://chromium-review.googlesource.com/1197404
Commit-Queue: Will Harris <wfh@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587858}

build/config/compiler/BUILD.gn

@@ -1762,10 +1762,36 @@ config("wexit_time_destructors") {
 # Note that this can be applied regardless of platform and architecture to
 # clean up the call sites. This will only apply the flag when necessary.
 #
+# This config is just an alias to no_shorten_64_warnings and will
+# suppress a superset of warning 4267 and any 64-bit -> 32-bit implicit
+# conversions. Having both for a time means not having to go through and
+# update all references to no_size_t_to_int_warning throughout the codebase
+# atomically.
+#
+# Any new warning suppressions should use the no_shorten_64_warnings
+# config below and not this.
+#
 # TODO(jschuh): crbug.com/167187 fix this and delete this config.
 config("no_size_t_to_int_warning") {
-  if (is_win && current_cpu == "x64") {
-    cflags = [ "/wd4267" ]
+  configs = [ ":no_shorten_64_warnings" ]
+}
+
+# As part of re-enabling -Wconversion (see issue 588506) some code
+# will continue to generate warnings.
+# The first warning to be enabled will be -Wshorten-64-to-32.
+#
+# Code that currently generates warnings for this can include this
+# config to disable them.
+config("no_shorten_64_warnings") {
+  if (current_cpu == "x64") {
+    if (is_clang) {
+      cflags = [ "-Wno-shorten-64-to-32" ]
+    } else {
+      # MSVC does not have an explicit warning equivalent to
+      # -Wshorten-64-to-32 but 4267 warns for size_t -> int
+      # on 64-bit builds, so is the closest.
+      cflags = [ "/wd4267" ]
+    }
   }
 }