enable WebContentsOcclusion on OS_WIN

Bug: 813093
Change-Id: Id7551c2b2f4bd0296203b035308aba57b0e5465e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1437895Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664581}

content/public/common/content_features.cc

@@ -566,7 +566,7 @@ const base::Feature kWebAssemblyTrapHandler{"WebAssemblyTrapHandler",
 // disabled, an occluded WebContents behaves exactly like a VISIBLE WebContents.
 const base::Feature kWebContentsOcclusion {
   "WebContentsOcclusion",
-#if defined(OS_MACOSX) || defined(OS_CHROMEOS)
+#if defined(OS_MACOSX) || defined(OS_CHROMEOS) || defined(OS_WIN)
       base::FEATURE_ENABLED_BY_DEFAULT
 #else
       base::FEATURE_DISABLED_BY_DEFAULT