Remove /fallback flag from 32-bit clang-cl invocations and enable -Werror

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 if (current_cpu == "mipsel" || current_cpu == "mips64el") {
@@ skipping 105 matching lines @@
         "/FIIntrin.h",
       ]
 
       if (visual_studio_version == "2013") {
         cflags += [ "-fmsc-version=1800" ]
       } else if (visual_studio_version == "2015") {
         cflags += [ "-fmsc-version=1900" ]
       }
 
       if (current_cpu == "x86") {
-        cflags += [
-          "/fallback",
-          "-m32",
-        ]
+        cflags += [ "-m32" ]
       } else {
         cflags += [ "-m64" ]
       }
       if (exec_script("//build/win/use_ansi_codes.py", [], "trim string") ==
           "True") {
         cflags += [
           # cmd.exe doesn't understand ANSI escape codes by default,
           # so only enable them if something emulating them is around.
           "-fansi-escape-codes",
         ]
@@ skipping 1095 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }