Enable Battery Status API by default in Blink.

Source/platform/RuntimeEnabledFeatures.in

 // http://dev.chromium.org/blink/runtime-enabled-features
 //
 // This list is used to generate RuntimeEnabledFeatures.h/cpp which contains
 // a class that stores static enablers for all experimental features.
 //
 // Each feature can be assigned a "status":
 // status=stable -> Enable this in all Blink configurations.  We are committed to these APIs indefinitely.
 // status=experimental -> In-progress features, Web Developers might play with, but are not on by default in stable.
 // status=test -> Enabled in ContentShell for testing, otherwise off.
 // status=deprecated -> Alias for "test", will be removed at some point.
 // Features without a status are not enabled anywhere by default.
 //
 // "stable" features listed here should be rare, as anything which we've shipped stable
 // can have its runtime flag removed soon after.
 //
 // condition=ENABLE_NAME is used for wrapping features in compile-time
 // #if ENABLE(FEATURE) guards.  These are deprecated and should all be removed.
 
 ApplicationCache status=stable
 AudioVideoTracks depends_on=Media, status=experimental
 AuthorShadowDOMForAnyElement
-BatteryStatus status=experimental
+BatteryStatus status=stable
 Beacon status=experimental
 
 // This feature is deprecated and we are evangalizing affected sites.
 // See https://crbug.com/346236 for current status.
 PrefixedVideoFullscreen status=stable
 
 // Enable bleeding-edge code to make Blink draw content faster.
 // The changes enabled behind this flag are very likely to break lots of content.
 // ** DO NOT use this flag unless you know what you are doing. **
 BleedingEdgeFastPaths
@@ skipping 118 matching lines @@
 WebGLDraftExtensions status=experimental
 WebGLImageChromium
 WebMIDI status=test
 WebVTTRegions depends_on=Media, status=experimental
 WOFF2 status=stable
 XSLT status=stable
 
 PseudoClassesInMatchingCriteriaInAuthorShadowTrees status=test
 
 CredentialManager status=test