Fix build with system yasm.

BUG=100078
Review URL: http://codereview.chromium.org/8243012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@105149 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/build/common.gypi b/build/common.gypi
index 9239aac..6c1d948 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -383,6 +383,9 @@
     # Whether to build for Wayland display server
     'use_wayland%': 0,
 
+    # Use system yasm instead of bundled one.
+    'use_system_yasm%': 0,
+
     # Default to enabled PIE; this is important for ASLR but we need to be
     # able to turn it off for remote debugging on Chromium OS
     'linux_disable_pie%': 0,