Add faster powf approximation for Android.

src/core/SkPaint.cpp

Index: src/core/SkPaint.cpp
diff --git a/src/core/SkPaint.cpp b/src/core/SkPaint.cpp
index 86b54f22c86531917a616b55e4379eaa7af77f4e..e5a55884d428dceee7c84a4aad08d6d2fa6ba646 100644
--- a/src/core/SkPaint.cpp
+++ b/src/core/SkPaint.cpp
@@ -1780,13 +1780,10 @@ void SkScalerContext::PostMakeRec(const SkPaint&, SkScalerContext::Rec* rec) {
             // use per-component information
 
             SkColor color = rec->getLuminanceColor();
-            U8CPU lum = SkColorSpaceLuminance::computeLuminance(rec->getPaintGamma(), color);
-            //If we are asked to look like LCD, look like LCD.
-            if (!(rec->fFlags & SkScalerContext::kGenA8FromLCD_Flag)) {
-                // HACK: Prevents green from being pre-blended as white.
-                lum -= ((255 - lum) * lum) / 255;
-            }
-
+            U8CPU lum = SkComputeLuminance(SkColorGetR(color),
+                                           SkColorGetG(color),
+                                           SkColorGetB(color));
+                                           
             // reduce to our finite number of bits
             color = SkColorSetRGB(lum, lum, lum);
             rec->setLuminanceColor(SkMaskGamma::CanonicalColor(color));