chromeos/aura: Handle power button on unofficial hardware.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index bc6b71bb715788b15071d13364961839b434184b..ced9808cd3b096a419c57da38a8996172c819286 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -642,6 +642,10 @@
     # Disable Dart by default.
     'enable_dart%': 0,
 
+    # (Most) Chrome OS hardware reports ACPI power button releases correctly.
+    # Standard hardware reports releases immediately after presses.
+    'chromeos_legacy_power_button%': 0,
+
     'conditions': [
       # Used to disable Native Client at compile time, for platforms where it
       # isn't supported (ARM)
@@ -1040,6 +1044,9 @@
       ['chromeos==1', {
         'defines': ['OS_CHROMEOS=1'],
       }],
+      ['chromeos_legacy_power_button==1', {
+        'defines': ['CHROMEOS_LEGACY_POWER_BUTTON=1'],
+      }],
       ['use_virtual_keyboard==1', {
         'defines': ['USE_VIRTUAL_KEYBOARD=1'],
       }],