Reverts "Skwasm single threaded (#56206)" (#56264)

Reverts: #56206
Initiated by: jonahwilliams
Reason for reverting: failing to roll into framework: flutter/flutter#157919 
Original PR Author: eyebrowsoffire

Reviewed By: {harryterkelsen}

This change reverts the following previous change:
This PR creates a single-threaded version of the skwasm renderer, appropriate for non-crossOriginIsolated browsing contexts.
* The single threaded renderer is essentially the same as the multi-threaded renderer, except instead of spawning a web worker and posting messages to it, it simply schedules microtasks on the main thread in their place.
* The new renderer is vended as `skwasm_st.js` and `skwasm_st.wasm` in the same location as multithreaded skwasm. In order to properly build and function, we needed some fixes I put into emscripten that landed in version 3.1.70. That version also changed some behavior that required a few fixes to the CanvasKit build files.
* The skwasm loader in flutter.js has been modified to use the skwasm_st variants when encountering a non-crossOriginIsolated context but a browser and configuration that otherwise would allow the use of skwasm. I also added a new `forceSingleThreadedSkwasm` option to the flutter configuration so that we can override this behavior, especially so that we can accurately benchmark the single threaded renderer in a crossOriginIsolated environment.
* I also consolidated a bunch of our shards that run tests to just have one per browser/platform combination, so four total. This will address flutter/flutter#124682

Author: auto-submit[bot]

DEPS

@@ -264,7 +264,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + '79bb5dfa69db3f72647c65d94dc1815bc84e9bed',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '3f1a1f0b26c22a3de5ffe0d3a081397550a12185',
 
   'src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',
@@ -762,7 +762,7 @@ deps = {
   },
 
   'src/flutter/prebuilts/emsdk': {
-   'url': Var('skia_git') + '/external/github.com/emscripten-core/emsdk.git' + '@' + '2514ec738de72cebbba7f4fdba0cf2fabcb779a5',
+   'url': Var('skia_git') + '/external/github.com/emscripten-core/emsdk.git' + '@' + 'a896e3d066448b3530dbcaa48869fafefd738f57',
    'condition': 'download_emsdk',
   },