Fix elliptical rrect clip shaders for large radii on devices with mediump

src/gpu/effects/GrOvalEffect.cpp

Index: src/gpu/effects/GrOvalEffect.cpp
diff --git a/src/gpu/effects/GrOvalEffect.cpp b/src/gpu/effects/GrOvalEffect.cpp
index 729e8a599c7bb8253813b935bac23082c20f89c5..3b373cbda7a31e4e8c3e404b9fd2d12fabcec83e 100644
--- a/src/gpu/effects/GrOvalEffect.cpp
+++ b/src/gpu/effects/GrOvalEffect.cpp
@@ -285,8 +285,9 @@ void GLEllipseEffect::emitCode(EmitArgs& args) {
     const EllipseEffect& ee = args.fFp.cast<EllipseEffect>();
     const char *ellipseName;
     // The ellipse uniform is (center.x, center.y, 1 / rx^2, 1 / ry^2)
+    // The last two terms can underflow on mediump, so we use highp.
     fEllipseUniform = args.fBuilder->addUniform(GrGLProgramBuilder::kFragment_Visibility,
-                                         kVec4f_GrSLType, kDefault_GrSLPrecision,
+                                         kVec4f_GrSLType, kHigh_GrSLPrecision,
                                          "ellipse",
                                          &ellipseName);