Don't use WPO for the Clang builds.

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")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 1237 matching lines @@
       # that some libraries aren't needed when they actually are,
       # http://crbug.com/234010. As workaround, disable --as-needed.
       common_optimize_on_ldflags += [ "-Wl,--as-needed" ]
     }
   }
 }
 
 # Default "optimization on" config.
 config("optimize") {
   if (is_win) {
-    if (is_official_build && full_wpo_on_official) {
+    # TODO(thakis): Remove is_clang here, https://crbug.com/598772
+    if (is_official_build && full_wpo_on_official && !is_clang) {
       common_optimize_on_cflags += [
         "/GL",  # Whole program optimization.
 
         # Disable Warning 4702 ("Unreachable code") for the WPO/PGO builds.
         # Probably anything that this would catch that wouldn't be caught in a
         # normal build isn't going to actually be a bug, so the incremental
         # value of C4702 for PGO builds is likely very small.
         "/wd4702",
       ]
     }
@@ skipping 198 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }