Suppress -Wshorten-64-to-32 warnings in gles2 generated code.

Also remove legacy /WD4267 and replace with the compiler config.

Warnings being suppressed are here: https://pastebin.com/raw/QkwwCZLb

BUG=879657

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: If145532f415cdcc4c52a48dc11f421171538b25d
Reviewed-on: https://chromium-review.googlesource.com/1199869Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588226}

gpu/command_buffer/client/BUILD.gn

@@ -103,7 +103,10 @@ source_set("gles2_cmd_helper_sources") {
     "gles2_cmd_helper_autogen.h",
   ]
 
-  configs += [ "//gpu:gpu_gles2_implementation" ]
+  configs += [
+    "//build/config/compiler:no_shorten_64_warnings",
+    "//gpu:gpu_gles2_implementation",
+  ]
 
   deps = [
     ":client",

gpu/command_buffer/common/BUILD.gn

@@ -124,7 +124,10 @@ source_set("gles2_sources") {
     "gles2_cmd_ids_autogen.h",
   ]
 
-  configs += [ "//gpu:gpu_gles2_implementation" ]
+  configs += [
+    "//gpu:gpu_gles2_implementation",
+    "//build/config/compiler:no_shorten_64_warnings",
+  ]
 
   deps = [
     ":gles2_utils",

gpu/gles2_conform_support/BUILD.gn

@@ -407,6 +407,7 @@ if (internal_gles2_conform_tests) {
     ]
     configs += [
       "//build/config/compiler:no_incompatible_pointer_warnings",
+      "//build/config/compiler:no_shorten_64_warnings",
 
       # Must be done this way for warning flags to be ordered correctly.
       ":gles2_conform_test_warnings",
@@ -436,7 +437,6 @@ if (internal_gles2_conform_tests) {
         "/wd4706",  # assignment within conditional expression
         "/wd4715",  # not all control paths return a value
         "/wd4716",  # function must return a value
-        "/wd4267",  # size_t/unsigned int conversion
       ]
     }
     if (is_mac) {