Add ability to query read pixels support without a render target.

src/gpu/gl/GrGLGpu.h

Index: src/gpu/gl/GrGLGpu.h
diff --git a/src/gpu/gl/GrGLGpu.h b/src/gpu/gl/GrGLGpu.h
index 4e7e4d6e14fedb159b4ced85a0c060278d81c84b..6f194dcd6c4296ebaebb0b1ec5ba86f6329e6d43 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -166,6 +166,20 @@ private:
     bool onMakeCopyForTextureParams(GrTexture*, const GrTextureParams&,
                                     GrTextureProducer::CopyParams*) const override;
 
+    // Checks whether glReadPixels can be called to get pixel values in readConfig from the
+    // render target.
+    bool readPixelsSupported(GrRenderTarget* target, GrPixelConfig readConfig);
+
+    // Checks whether glReadPixels can be called to get pixel values in readConfig from a
+    // render target that has renderTargetConfig. This may have to create a temporary
+    // render target and thus is less preferable than the variant that takes a render target.
+    bool readPixelsSupported(GrPixelConfig renderTargetConfig, GrPixelConfig readConfig);
+
+    // Checks whether glReadPixels can be called to get pixel values in readConfig from a
+    // render target that has the same config as surfaceForConfig. Calls one of the the two
+    // variations above, depending on whether the surface is a render target or not.
+    bool readPixelsSupported(GrSurface* surfaceForConfig, GrPixelConfig readConfig);
+
     bool onReadPixels(GrSurface*,
                       int left, int top,
                       int width, int height,