aura: Have aura and aura_demo build and run without gtk on linux.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 5b77b1cd59840e74e5e1d9e09428baa8571cae0a..045cca20409e5986d97b7d2cad0f87bcd87c81d8 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -237,7 +237,7 @@
           'use_x11%': 1,
         }],
         ['use_aura==1 and OS!="win"', {
-          'toolkit_uses_gtk%': 1,
+          'toolkit_uses_gtk%': 0,
         }],
 
         # A flag to enable or disable our compile-time dependency
@@ -1097,11 +1097,17 @@
                           ['exclude', '(^|/)(cocoa|mac)/'],
                           ['exclude', '\\.mm?$' ] ],
           }],
+          ['use_x11!=1', {
+            'sources/': [
+              ['exclude', '_(chromeos|x|x11)(_unittest)?\\.(h|cc)$'],
+              ['exclude', '(^|/)x11_[^/]*\\.(h|cc)$'],
+            ],
+          }],
           ['toolkit_uses_gtk!=1', {
             'sources/': [
-              ['exclude', '_(chromeos|gtk|x|x11|xdg)(_unittest)?\\.(h|cc)$'],
+              ['exclude', '_(gtk|xdg)(_unittest)?\\.(h|cc)$'],
               ['exclude', '(^|/)gtk/'],
-              ['exclude', '(^|/)(gtk|x11)_[^/]*\\.(h|cc)$'],
+              ['exclude', '(^|/)gtk_[^/]*\\.(h|cc)$'],
             ],
           }],
           ['use_wayland!=1', {