ozone: Remove GLImageNativePixmap log spam.

GLImageNativePixmap is currently logging "Failed to flush rendering" for
each frame on ARM devices.

The log is harmless, but the underlying cause of failure should be eventually
investigated.

This CL removes the log statement in release builds.

Bug: b/63364517
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I47eefd31eb02893856248f64f338fbcb78666922
Reviewed-on: https://chromium-review.googlesource.com/764670
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515797}

ui/gl/gl_image_native_pixmap.cc

@@ -248,7 +248,9 @@ void GLImageNativePixmap::Flush() {
       EGL_NONE,
   };
   if (!eglImageFlushExternalEXT(display, egl_image_, attribs)) {
-    LOG(ERROR) << "Failed to flush rendering";
+    // TODO(reveman): Investigate why we're hitting the following log
+    // statement on ARM devices. b/63364517
+    DLOG(WARNING) << "Failed to flush rendering";
     return;
   }
 }