The Open Source WebKit part is significantly less self-contained than Firefox or Chromium. A whole lot of stuff that Firefox or Chrome implement themselves relies on OS X system APIs in the case of Safari. (Some of those things may be open sourced separately, though.) Non-OS X WebKit ports have to implement all that stuff on their own on top of whatever infrastructure they use, such as Qt.

For Chrome:

Everyone knows about the PPAPI Flash Player.

Additionally, https://src.chromium.org/svn/trunk/src/build/all.gyp gives hints about other stuff that's built in Chrome but not in Chromium. Some of that stuff is actually Open Source. For example, courgette shows up on the Chrome-only list, but a quick search suggests that it's Open Source.

However, in that file, you can find references to Widevine (DRM) and the PDF reader without finding source code for either in the repo.

Also, there's some Chrome-only print stuff and it's not clear how much of that is Open Source and how much proprietary and if it is Open Source, why it's not built in Chromium.

The stuff in all.gyp is not the whole story, like the can concluded from knowledge about Flash Player without seeing it mentioned there.

To find out more, one would need to go through https://src.chromium.org/svn/trunk/src/third_party/ and figure out which dependencies aren't Open Source. In addition to the adobe/ directory there, you can see swiftshader/ (software fallback renderer for WebGL). It comes with a README.chromium file (https://src.chromium.org/svn/trunk/src/third_party/swiftshad...) that says: "License: Proprietary".

So that gives a minimum of:

* Flash Player

* PDF Reader

* Widevine

* SwiftShader

* Maybe some Cloud Print stuff.

(Edit: typo and formatting)