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Currently Piex wasm loads into a global window.Module object as a side-
effect of importing the wasm JS glue. This approach may inhibit other
wasm modules loading into the same JS context.

To fix, configure emscripten to wrap the Piex module in a loader
function that returns a promise, and capture it into a unique identifier
on initialization.

This approach lends itself to on-demand loading of the wasm, so now the
wasm subsystem is only initialized when required, rather than whenever
the image loader extension starts up. That is, on the first request to
refresh the thumbnail for a RAW image, which should occur rarely.

Re-ran the Makefile at emscripten 1.38.33 (same as r815044), and ensured
that my local sdk created identical files before adding MODULARIZE flags.

Ran the separate test.sh. Test coverage also provided in browser_tests.

Bug: b/175505626
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: I533a8d8028f6913efe0739c5e5cf80864addbe0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584634Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836985}