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Piex is the "Preview Image EXtractor" and is used by the Files app on
ChromeOS for creating thumbnails inside the "image loader" component
extension. It uses the `wasm-eval` CSP directive, which is only
currently enabled for apps and extensions (standardisation being
discussed at github.com/WebAssembly/content-security-policy/pull/13 ).

To make the WASM work, this CL enables `wasm-eval` for
chrome-untrusted://* CSP directives, and adds that to
chrome-untrusted://media-app's CSP.

Two new JS files are added to chrome-untrusted://media-app:
 - piex_module.js is a thin wrapper around the existing piex_loader.js
   to do (effectively) RAW -> JPEG conversions of Blob data.
 - piex_module_loader.js adds logic for runtime-loading of the
   WebAssembly, piex_loader.js, and piex_module.js

An end-to-end test using the handcrafted "raw.orf" file is added to
ensure the WASM loads and successfully extracts preview data, and it
is successfully loaded by the app.

Note this CL adds support, but doesn't change file handlers to direct
RAW files to chrome://media-app, which will be gated behind a flag.

Design Doc: go/bl-raw
Launch Bug: https://crbug.com/1126203

Bug: 1030935, b/154062029, b/167496867
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: I0c1f1da8bc39c8e6223214b49d9ac09dc93bf420
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422100Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Patti <patricialor@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Auto-Submit: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818636}