[piexwasm] Add FilesApp PiexWasm feature flag

Add ChromeOS --enable-features=PiexWasm flag, disabled-by-default. Add
a chrome about:flags entry. Change the file manager private API

  chrome.fileManagerPrivate.isPiexLoaderEnabled()

to return whether to use 'piex-nacl' to handle raw images in the Image
Loader extension noting that 'piex-nacl' is only available in official
builds [1].

[1] Seems odd, but it is sensible since the PiexNaCl module resides in
an external repository. The PiexWasm module is present in the chromium
repository (this so we can remove the private API, and official-build-
only limitation, in future).

Bug: 904630
Change-Id: If5884b79708c10bcfb1090eb25adc299a5949f64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610263
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659381}
8 files changed