Fix warning introduced in "Program::getUniformInternal: return only one array element"

The warning does not seem to occur on the Chromium bots, but shows
when compiling ANGLE standalone.

BUG=595836

Change-Id: I3c22bbea263223f9e92f82229817e9e9894a46ad
Reviewed-on: https://chromium-review.googlesource.com/335576
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/tests/gl_tests/UniformTest.cpp

@@ -601,7 +601,7 @@ TEST_P(UniformTestES3, ReturnsOnlyOneArrayElement)
         "    color = vec4(1, 0, 0, 1);\n"
         "}";
 
-    GLuint mProgram = CompileProgram(vertexShader, fragmentShader);
+    mProgram = CompileProgram(vertexShader, fragmentShader);
     ASSERT_NE(0u, mProgram);
 
     glUseProgram(mProgram);