Wait for GL command completion in the ExternalTextureGLRefreshedTooOften test (#42438)

This test sometimes caused assertion failures when running on Windows with ANGLE.  The process may be unable to safely clean up global objects if GL commands are pending when the test exits.

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,