After https://crrev.com/c/612117, session_manager runs Chrome with --enable-native-gpu-memory-buffers if the native_gpu_memory_buffers USE flag is set and --disable-native-gpu-memory-buffers if it isn't.

After https://crrev.com/c/551365, Chrome has the following behavior:

- For x86 Chrome OS, it enables native buffers by default and disables them if --disable-native-gpu-memory-buffers was passed.

- For non-x86 Chrome OS, it disables native buffers by default and enables them if --enable-native-gpu-memory-buffers was passed.

This is way too complicated. Now we have --enable and --disable flags that are ignored in some cases and a USE flag that's set inconsistently. The logic to enable and disable features shouldn't be split across multiple places like this; it makes it very hard to figure out what's going on.

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I think that we should instead use something like the following:

- Chrome should always disable native buffers by default for all Chrome OS devices and enable them only when the --enable flag is passed (i.e. revert https://crrev.com/c/551365).

- The native_gpu_memory_buffers USE flag should be set for all boards that support native buffers.

- session_manager should pass --enable-native-gpu-memory-buffers if the native_gpu_memory_buffers USE flag is set and never pass the --disable flag.

(If the eventual plan is to enable native buffers on all boards, then feel free to reverse the above logic: Chrome enables by default and checks --disable, the USE flag is set by default and unset on boards that don't support native buffers, and session manager passes --disable when the USE flag isn't set.)