Use highp when necessary in shader-precision-format. (#3743)

The test is written against WebGL 1.0, where it's legal for
implementations to not support highp in the fragment shader. Use highp
for tests of highp integer values, but continue to use mediump for
tests of both lowp and mediump ints.

Fixes #3739.

Author: kenrussell

sdk/tests/conformance/misc/shader-precision-format.html

@@ -223,6 +223,9 @@
     testRenderPrecision(gl, shaderType, 'int', precision, expected, msg);
   }
 
+  // Only use highp in the fragment shader if caller has determined it's supported.
+  const uniformPrecision = (precision == 'highp' ? 'highp' : 'mediump');
+
   {
     const vs = `
     attribute vec4 position;
@@ -261,7 +264,7 @@
     const fs = `
     precision ${precision} float;
     uniform ${precision} int v;
-    uniform mediump float f;
+    uniform ${uniformPrecision} float f;
 
     void main() {
       mediump float diff = abs(float(v) - f);