Wait for GL command completion in the ExternalTextureGLRefreshedTooOften test

This test caused assertion failures when running on Windows with ANGLE if GL
commands are pending when the test exits and the process shuts down.

Author: jason-simmons

shell/platform/embedder/tests/embedder_gl_unittests.cc

@@ -4019,10 +4019,14 @@ TEST_F(EmbedderTest, ExternalTextureGLRefreshedTooOften) {
   auto context = surface.GetGrContext();
 
   typedef void (*glGenTexturesProc)(uint32_t n, uint32_t* textures);
+  typedef void (*glFinishProc)();
+
   glGenTexturesProc glGenTextures;
+  glFinishProc glFinish;
 
   glGenTextures = reinterpret_cast<glGenTexturesProc>(
       surface.GetProcAddress("glGenTextures"));
+  glFinish = reinterpret_cast<glFinishProc>(surface.GetProcAddress("glFinish"));
 
   uint32_t name;
   glGenTextures(1, &name);
@@ -4067,6 +4071,8 @@ TEST_F(EmbedderTest, ExternalTextureGLRefreshedTooOften) {
                   DlImageSampling::kLinear);
 
   EXPECT_TRUE(resolve_called);
+
+  glFinish();
 }
 
 TEST_F(EmbedderTest,