Make ANGLE perf decisions be runtime rather than compile time

R=robertphillips@google.com

Review URL: https://codereview.chromium.org/1268953002
diff --git a/src/gpu/gl/GrGLCaps.h b/src/gpu/gl/GrGLCaps.h
index 0622e68..01ce5bb 100644
--- a/src/gpu/gl/GrGLCaps.h
+++ b/src/gpu/gl/GrGLCaps.h
@@ -281,6 +281,9 @@
 
     LATCAlias latcAlias() const { return fLATCAlias; }
 
+    bool rgba8888PixelsOpsAreSlow() const { return fRGBA8888PixelsOpsAreSlow; }
+    bool partialFBOReadIsSlow() const { return fPartialFBOReadIsSlow; }
+
     GrGLSLCaps* glslCaps() const { return reinterpret_cast<GrGLSLCaps*>(fShaderCaps.get()); }
 
 private:
@@ -380,6 +383,8 @@
     bool fFullClearIsFree : 1;
     bool fBindFragDataLocationSupport : 1;
     bool fSRGBWriteControl : 1;
+    bool fRGBA8888PixelsOpsAreSlow : 1;
+    bool fPartialFBOReadIsSlow : 1;
 
     struct ReadPixelsSupportedFormat {
         GrGLenum fFormat;