Add helper comment for icecc users.

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 720 matching lines @@
           'linux_use_bundled_gold%': 1,
         }, {
           'linux_use_bundled_gold%': 0,
         }],
 
         # linux_use_bundled_binutils: whether to use the binary binutils
         # checked into third_party/binutils.  These are not multi-arch so cannot
         # be used except on x86 and x86-64 (the only two architectures which
         # are currently checke in).  Force this off via GYP_DEFINES when you
         # are using a custom toolchain and need to control -B in cflags.
+        # Set to 1 when using icecc to avoid incompatible build flags.
+        # https://github.com/icecc/icecream/commit/b2ce5b9cc4bd1900f55c3684214e409fa81e7a92
         ['OS=="linux" and (target_arch=="x64")', {
           'linux_use_bundled_binutils%': 1,
         }, {
           'linux_use_bundled_binutils%': 0,
         }],
 
         # linux_use_gold_flags: whether to use build flags that rely on gold.
         # On by default for x64 Linux.  Temporarily off for ChromeOS as
         # it failed on a buildbot.
         ['OS=="linux" and target_arch=="x64" and chromeos==0', {
@@ skipping 4478 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }