Use PCH by default in VS 2010.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 154b94786ca9e3daaf62f6b0188e247e83fbccd1..755d52649725c2bf8ba2c76ceffe5704510542d8 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -251,11 +251,23 @@
       # For example, use_xi2_mt=2 means XI2.2 or above version is required.
       'use_xi2_mt%': 0,
 
-      # Use of precompiled headers on Windows is off by default
-      # because of complications that it can cause with our
-      # infrastructure (trybots etc.).  Enable by setting to 1 in
-      # ~/.gyp/include.gypi or via the GYP command line for ~20-25%
-      # faster builds.
+      # Use of precompiled headers on Windows.
+      #
+      # This is on by default in VS 2010, but off by default for VS
+      # 2008 because of complications that it can cause with our
+      # trybots etc.
+      #
+      # This variable may be explicitly set to 1 (enabled) or 0
+      # (disabled) in ~/.gyp/include.gypi or via the GYP command line.
+      # This setting will override the default.
+      #
+      # Note that a setting of 1 is probably suitable for most or all
+      # Windows developers using VS 2008, since precompiled headers
+      # provide a build speedup of 20-25%.  There are a couple of
+      # small workarounds you may need to use when using VS 2008 (but
+      # not 2010), see
+      # http://code.google.com/p/chromium/wiki/WindowsPrecompiledHeaders
+      # for details.
       'chromium_win_pch%': 0,
 
       'conditions': [
@@ -365,6 +377,11 @@
         }, {
           'ui_compositor_image_transport%': 0,
         }],
+
+        # Turn precompiled headers on by default for VS 2010.
+        ['OS=="win" and MSVS_VERSION=="2010"', {
+          'chromium_win_pch%': 1
+        }],
       ],
     },