gpu: Don't clear texture level in DoCopyTexSubImage2D() if not needed.

gpu/command_buffer/service/gles2_cmd_decoder.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.cc b/gpu/command_buffer/service/gles2_cmd_decoder.cc
index 0d778f3570ed2ab036adb03d327847c825248b4c..f56d47bad381a31ed09cf508a1ba09d9314b97b6 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -9145,10 +9145,17 @@ void GLES2DecoderImpl::DoCopyTexSubImage2D(
   Clip(x, width, size.width(), &copyX, &copyWidth);
   Clip(y, height, size.height(), &copyY, &copyHeight);
 
-  if (!texture_manager()->ClearTextureLevel(this, texture_ref, target, level)) {
-    LOCAL_SET_GL_ERROR(
-        GL_OUT_OF_MEMORY, "glCopyTexSubImage2D", "dimensions too big");
-    return;
+  if (xoffset != 0 || yoffset != 0 || width != size.width() ||
+      height != size.height()) {
+    if (!texture_manager()->ClearTextureLevel(this, texture_ref, target,
+                                              level)) {
+      LOCAL_SET_GL_ERROR(GL_OUT_OF_MEMORY, "glCopyTexSubImage2D",
+                         "dimensions too big");
+      return;
+    }
+  } else {
+    // Write all pixels in below.
+    texture_manager()->SetLevelCleared(texture_ref, target, level, true);
   }
 
   if (copyX != x ||