Add MAX_TEXTURE_IMAGE support to gpu_image_decode test gles interface.

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I67e0f6017e84d964af316cef1f912dd58d050ce4
Reviewed-on: https://chromium-review.googlesource.com/c/1264922Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: Greg Daniel <egdaniel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597545}

cc/tiles/gpu_image_decode_cache_unittest.cc

@@ -185,6 +185,9 @@ class FakeGPUImageDecodeTestGLES2Interface : public viz::TestGLES2Interface,
   }
   void GetIntegerv(GLenum name, GLint* params) override {
     switch (name) {
+      case GL_MAX_TEXTURE_IMAGE_UNITS:
+        *params = 8;
+        return;
       case GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS:
         *params = 8;
         return;