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Wrap raw image data in a PiexStreamReader, and process it via Piex, to
extract the preview and thumbnail image properties.

Caller (JS on the same web page) owns the data. All this code needs to
do is provide the offset and length of the image preview or thumbnail,
as well its EXIF color space and orientation information, and callback
these results in a JS object. The result includes the camera maker and
model, and preview and thumbnail image format/width/height properties,
for testing purposes [1].

Add fake DEPS to node_modules/piex to silence bogus presubmit error on
uploading this patch. Update Wasm Makefile for new piex.cpp code.

Build products we care about are the a.out.{js,wasm}. Build sizes look
good running:

  % ls -sh a.out*
  36K a.out.js
  60K a.out.wasm

[1] Note the upstream Piex code is fuzzed, and has a large test suite.
Our test approach can be much simpler therefore, as we'll soon see.

Tbr: slangley
Bug: 935285
Change-Id: I406dc08c908027782390e793bf6c7765bf0afa39
Reviewed-on: https://chromium-review.googlesource.com/c/1488474
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635788}