system_api: Add power_manager::kSetScreenBrightnessMethod.

Add a new "SetScreenBrightness" constant. The existing
powerd SetScreenBrightnessPercent is being updated to take a
protocol message instead of D-Bus args, so we might as well
rename it to be shorter at the same time.

BUG=chromium:881786
TEST=used with updated powerd

Change-Id: I720b259fe518e0a9a45f981a0ba1db68d9a726b4
Reviewed-on: https://chromium-review.googlesource.com/1242034
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
diff --git a/dbus/power_manager/backlight.proto b/dbus/power_manager/backlight.proto
index 03bde65..5ceb19b 100644
--- a/dbus/power_manager/backlight.proto
+++ b/dbus/power_manager/backlight.proto
@@ -14,7 +14,7 @@
 // hardware levels.
 
 // Request to change the backlight brightness sent from Chrome to powerd in a
-// SetScreenBrightnessPercent D-Bus method call.
+// SetScreenBrightness D-Bus method call.
 message SetBacklightBrightnessRequest {
   // Desired backlight brightness as a percent in the range [0.0, 100.0].
   optional double percent = 1;
diff --git a/dbus/power_manager/dbus-constants.h b/dbus/power_manager/dbus-constants.h
index 9159da9..580db56 100644
--- a/dbus/power_manager/dbus-constants.h
+++ b/dbus/power_manager/dbus-constants.h
@@ -13,9 +13,11 @@
 const char kPowerManagerServiceName[] = "org.chromium.PowerManager";
 
 // Methods exposed by powerd.
+const char kSetScreenBrightnessMethod[] = "SetScreenBrightness";
 const char kDecreaseScreenBrightnessMethod[] = "DecreaseScreenBrightness";
 const char kIncreaseScreenBrightnessMethod[] = "IncreaseScreenBrightness";
 const char kGetScreenBrightnessPercentMethod[] = "GetScreenBrightnessPercent";
+// TODO(derat): Delete this after callers are using SetScreenBrightness.
 const char kSetScreenBrightnessPercentMethod[] = "SetScreenBrightnessPercent";
 const char kGetKeyboardBrightnessPercentMethod[] =
     "GetKeyboardBrightnessPercent";