Enable sRGB on iOS, make sRGB decode support optional

src/gpu/gl/GrGLCaps.h

Index: src/gpu/gl/GrGLCaps.h
diff --git a/src/gpu/gl/GrGLCaps.h b/src/gpu/gl/GrGLCaps.h
index 5c7e02874575296540d2a78e56d82b377724a505..c0ba7f51c595f6915a2e73c4acad20b5fc1c699d 100644
--- a/src/gpu/gl/GrGLCaps.h
+++ b/src/gpu/gl/GrGLCaps.h
@@ -345,6 +345,8 @@ public:
 
     bool doManualMipmapping() const { return fDoManualMipmapping; }
 
+    bool srgbDecodeSupport() const { return fSRGBDecodeSupport; }
+
     /**
      * Returns a string containing the caps info.
      */
@@ -420,6 +422,8 @@ private:
     bool fMipMapLevelAndLodControlSupport : 1;
     bool fRGBAToBGRAReadbackConversionsAreSlow : 1;
     bool fDoManualMipmapping : 1;
+    bool fSRGBDecodeSupport : 1;
+    bool fRequireDecodeForSRGB : 1;
 
     uint32_t fBlitFramebufferFlags;