Cached references to the WASM heap buffer are invalid following memory growth:
https://github.com/emscripten-core/emscripten/issues/6747#issuecomment-400081465.
This change replaces references to the cached CanvasKit.buffer with direct
references to CanvasKit.HEAPU8.buffer. The symptom of this bug is a Javascript
error thrown in the Chrome console: "Uncaught TypeError: Cannot perform
Construct on a neutered ArrayBuffer", causing the operation in question to fail.
Bug: NONE
Change-Id: I27462e80db1d33e0e77ee7295b25039c9036d2e2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/264477
Reviewed-by: Kevin Lubick <kjlubick@google.com>