Use precompiled headers for most large projects.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index a79a8a07b908ef73bbeb7b1a0de8e5d3d3e5ac32..203bb6a48fc42024e438f9bb27686c8fc28364b1 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -197,6 +197,9 @@
       # Webrtc compilation is enabled by default. Set to 0 to disable.
       'enable_webrtc%': 1,
 
+      # Whether to use precompiled headers on Windows.
+      'use_precompiled_headers%': 0,
+
       'conditions': [
         # Use Skia as WebKit renderer on Mac
         ['OS=="mac"', {
@@ -302,6 +305,7 @@
     'use_third_party_translations%': '<(use_third_party_translations)',
     'remoting%': '<(remoting)',
     'enable_webrtc%': '<(enable_webrtc)',
+    'use_precompiled_headers%': '<(use_precompiled_headers)',
     'p2p_apis%': '<(p2p_apis)',
     'configuration_policy%': '<(configuration_policy)',
     'safe_browsing%': '<(safe_browsing)',
@@ -817,6 +821,15 @@
           }],
         ],  # conditions for fastbuild.
       }],  # fastbuild!=0
+      ['use_precompiled_headers!=0', {
+        'conditions': [
+	  ['OS=="win"', {
+            'sources': ['<(DEPTH)/build/precompile.cc'],
+            'msvs_precompiled_header': '<(DEPTH)/build/precompile.h',
+            'msvs_precompiled_source': '<(DEPTH)/build/precompile.cc',
+  	  }],  # OS=="win"
+	],  # conditions
+      }],  # use_precompiled_headers!=0
       ['selinux==1', {
         'defines': ['CHROMIUM_SELINUX=1'],
       }],