Provide v8 dependency on tracing.

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 1266 matching lines @@
     'cfi_diag%': '<(cfi_diag)',
     'cfi_blacklist%': '<(cfi_blacklist)',
     'mac_views_browser%': '<(mac_views_browser)',
     'android_app_version_name%': '<(android_app_version_name)',
     'android_app_version_code%': '<(android_app_version_code)',
     'use_webview_internal_framework%': '<(use_webview_internal_framework)',
     'enable_webvr%': '<(enable_webvr)',
 
     # Turns on compiler optimizations in V8 in Debug build.
     'v8_optimized_debug%': 1,
+    'v8_tracing_include_dir': '<(DEPTH)',

[Michael Achenbach, 2016/06/17 06:41:03]

nit: It might be minimally confusing that there's no empty line between the
lines and the one above has a comment. It looks like the things belong together
but they don't.

 
     # Use system protobuf instead of bundled one.
     'use_system_protobuf%': 0,
 
     # Use system yasm instead of bundled one.
     'use_system_yasm%': 0,
 
     # Use system ICU instead of bundled one.
     'use_system_icu%' : 0,
 
@@ skipping 5064 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',
   },
 }