Make base::PowerMonitor work on Chrome OS.

base/base.gypi

Index: base/base.gypi
diff --git a/base/base.gypi b/base/base.gypi
index 8a37249397cc03bce57aa0b2da263246bc66d1b9..c5796ef8c1655663549e7754c14493670da0f1f3 100644
--- a/base/base.gypi
+++ b/base/base.gypi
@@ -432,6 +432,7 @@
           'power_monitor/power_monitor.h',
           'power_monitor/power_monitor_device_source_android.cc',
           'power_monitor/power_monitor_device_source_android.h',
+          'power_monitor/power_monitor_device_source_chromeos.cc',
           'power_monitor/power_monitor_device_source.cc',
           'power_monitor/power_monitor_device_source.h',
           'power_monitor/power_monitor_device_source_ios.mm',
@@ -838,6 +839,11 @@
                'ANDROID_SINCOS_PROVIDED',
             ],
           }],
+          ['<(chromeos) == 1', {

[Lei Zhang, 2014/10/06 19:09:28]

In most gyp files we just write chromeos==1. Why do we have to do it in the
early phase here?

[Daniel Erat, 2014/10/06 21:44:13]

i tried that initially, but it produces an error saying that the chromeos
identifier is undefined:

gyp: name 'chromeos' is not defined while evaluating condition 'chromeos == 1'
in /derat/chrome/src/base/base.gyp

in the existing target_condition expressions in this file, pretty much all
user-defined variables (chromeos, desktop_linux, use_glib, os_bsd, etc.) appear
to be using the early/pre-expansion syntax.

my initial thought would be that these variables are only defined after
base.gypi is included in base.gyp, but it doesn't make sense to me why early
expansion would help then (and i get the same "not defined" error when i use
late expansion, oddly). any idea what's going on here?

+            'sources!': [
+              'power_monitor/power_monitor_device_source_posix.cc',
+            ],
+          }],
           ['OS == "ios" and _toolset != "host"', {
             'sources/': [
               # Pull in specific Mac files for iOS (which have been filtered out