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This establishes the framework through which the WebAssembly-backed web
worker will send tree updates to the viewer thread.

Currently WebAssembly constructs a JSON object, writes it to a char* in
heap memory, passes that pointer to JavaScript, which reads the string
out of heap memory, parses it into a JS object, and frees the heap
pointer. Since sending the JS object from the web worker to the viewer
thread involves a serialization/deserialization, this is performing an
extra serialization/deserialization, but because any given tree load is
only loading a 1-deep view of the file tree, the performance hit is very
small. This sacrifice lets us maintain a common web worker API between
the wasm and non-wasm implementations.

TODOs have been liberally sprinkled through the CL reflecting future
functionality like node filtering, components, and type-based size
breakdowns.

Bug: 1011921
Change-Id: I383ec97bf252657c8840d0e36cb99f52645116c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1873108Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709479}