Stop using GradientShaderCache for gradient textures

Ganesh uses GradientShaderCache for generating legacy gradient textures,
and a custom helper (initLinearBitmap) for all other cases.

Now that GradientShaderCache no longer supports dithering, there is
little difference between the two implementantions.  And sice the result
is also cached in a separate bitmap cache, we might as well consolidate
the implementation and always use initLinearBitmap.

This allows us to completely remove GradientShaderCache in a follow up.

Change-Id: Ie05d15132a6116d2f139f066b81fcfbc9e22d7b6
Reviewed-on: https://skia-review.googlesource.com/63741
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/src/shaders/gradients/SkGradientShaderPriv.h b/src/shaders/gradients/SkGradientShaderPriv.h
index 26bc033..3a181dd 100644
--- a/src/shaders/gradients/SkGradientShaderPriv.h
+++ b/src/shaders/gradients/SkGradientShaderPriv.h
@@ -188,7 +188,7 @@
 
     bool onAsLuminanceColor(SkColor*) const override;
 
-    void initLinearBitmap(SkBitmap* bitmap) const;
+    void initLinearBitmap(SkBitmap* bitmap, GradientBitmapType) const;
 
     bool onAppendStages(const StageRec&) const override;