[canvaskit] POC bindings for testing gms

In this CL, I forked compile.sh and created a new gm_bindings.cpp.
I also moved viewer.html into wasm_tools and created a gmtests.html
for testing out the bindings locally.

Right now there is only one gm file compiled in. I plan in a followup
CL to have some way to generate the list of cpp files that need to
be compiled in from gms.gni. I was unable to get it to work with
simply linking the lib_gm.gni, probably due to the same issue with
Registry that csmartdalton@ ran into when adding viewer.html
and the associated bindings.

Suggested reviewing order:
 - gmtests.html to get a sense of how the test flow works.
 - gm_bindings.cpp to make sure I setup the contexts/GMs correctly.
 - compile_gm.sh to see how the gms are compiled in.
 - The remaining files in any order.

When I tested this locally, the bleed_downscale digest was
exactly the same (pixel for pixel, byte for byte) as a known
digest in Gold, so I'm fairly confident in how things work.

Change-Id: I2babef848ca60f7db74e4adf27b8952a66bdeee1
Bug: skia:10812
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/322956
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
9 files changed