aura: Use WebKit compositor by default

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index fcd3d3fb8a9d192db6bbc1971ae85363954bd8ec..bb3726c6c50d49eea1a133c94535b8638d393be8 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -51,9 +51,6 @@
         'use_virtual_keyboard%': '<(use_virtual_keyboard)',
         'use_skia_on_mac%': '<(use_skia_on_mac)',
 
-        # WebKit compositor for ui
-        'use_webkit_compositor%': 0,
-
         # Compute the architecture that we're building on.
         'conditions': [
           [ 'OS=="win" or OS=="mac"', {
@@ -77,6 +74,13 @@
           ['use_aura==1', {
             'views_compositor%': 1,
           }],
+
+          # Use the WebKit compositor for ui, when Aura is on.
+          ['use_aura==1', {
+            'use_webkit_compositor%': 1,
+          }, {
+            'use_webkit_compositor%': 0,
+          }],
         ],
       },