Add ANGLE workaround to prefer flushes over VRAM usage

On the whole, https://codereview.chromium.org/1286203002/ (Defer flushes if kPreferNoIO is specified) improved performance but it did cause a performance regression on ANGLE. This CL disables the deferral of flushes on ANGLE until we can add a separate incremental flushing mechanism.

TBR=bsalomon@google.com

BUG=skia:4201
BUG=521529

Review URL: https://codereview.chromium.org/1287193008
diff --git a/src/gpu/gl/GrGLCaps.cpp b/src/gpu/gl/GrGLCaps.cpp
index 21d3d93..af0fa78 100644
--- a/src/gpu/gl/GrGLCaps.cpp
+++ b/src/gpu/gl/GrGLCaps.cpp
@@ -471,6 +471,11 @@
         fUseDrawInsteadOfPartialRenderTargetWrite = true;
     }
 
+#ifdef SK_BUILD_FOR_WIN
+    // On ANGLE deferring flushes can lead to GPU starvation
+    fPreferVRAMUseOverFlushes = !isANGLE;
+#endif
+
     if (kChromium_GrGLDriver == ctxInfo.driver()) {
         fMustClearUploadedBufferData = true;
     }
@@ -1147,8 +1152,8 @@
     r.appendf("Use non-VBO for dynamic data: %s\n",
              (fUseNonVBOVertexAndIndexDynamicData ? "YES" : "NO"));
     r.appendf("SRGB write contol: %s\n", (fSRGBWriteControl ? "YES" : "NO"));
-    r.appendf("RGBA 8888 pixel ops are slow: %s\n", (fRGBA8888PixelsOpsAreSlow? "YES" : "NO"));
-    r.appendf("Partial FBO read is slow: %s\n", (fPartialFBOReadIsSlow? "YES" : "NO"));
+    r.appendf("RGBA 8888 pixel ops are slow: %s\n", (fRGBA8888PixelsOpsAreSlow ? "YES" : "NO"));
+    r.appendf("Partial FBO read is slow: %s\n", (fPartialFBOReadIsSlow ? "YES" : "NO"));
     return r;
 }