Stop explicitly disabling SharedArrayBuffers

[fmarier]

SharedArrayBuffers were explicitly disabled in brave/brave-core#114 in order to match the Muon settings in the MVP.

Given that Google is actively addressing the Spectre threat, I think we can remove our customization and rely on their judgement when it comes to re-enabling it and the extra protections that would be needed for that to happen.

[fmarier]

Testing this is really tricky.

I found an old test page with some code to check for SharedArraybuffers:

  if (typeof SharedArrayBuffer !== 'function') {
    alert('this browser does not have SharedArrayBuffer support enabled' + '\n\n' + msg);
    return
  }

However, the problem is that now the SharedArrayBuffer constructor is now always available.

So I instead wrote a test page which passes a SharedArrayBuffer via postMessage to a worker. The problem however is that it works just fine in release without my change. That's because the flag we are setting is overridden later by SetCustomizedRuntimeFeaturesFromCombinedArgs(). If I disable that override:

--- a/content/child/runtime_features.cc
+++ b/content/child/runtime_features.cc
@@ -537,9 +537,9 @@ void SetCustomizedRuntimeFeaturesFromCombinedArgs(
     WebRuntimeFeatures::EnableDecodeLossyWebPImagesToYUV(true);
   }
 
-  WebRuntimeFeatures::EnableSharedArrayBuffer(
-      base::FeatureList::IsEnabled(features::kSharedArrayBuffer) ||
-      base::FeatureList::IsEnabled(features::kWebAssemblyThreads));
+  WebRuntimeFeatures::EnableSharedArrayBuffer(false);
+//      base::FeatureList::IsEnabled(features::kSharedArrayBuffer) ||
+//      base::FeatureList::IsEnabled(features::kWebAssemblyThreads));
 
   // These checks are custom wrappers around base::FeatureList::IsEnabled
   // They're moved here to distinguish them from actual base checks

then I can confirm that our code is correctly disabling the feature.

Furthermore, my test page specifically avoids the isolation headers because the feature is also enabled for cross-origin isolated renderers regardless of the feature flag. Howevever, while it's a good test right now, it will stop working in C91.

The alternative, which will work in C91 or later, is to apply this hack:

--- a/third_party/blink/renderer/core/execution_context/execution_context.cc
+++ b/third_party/blink/renderer/core/execution_context/execution_context.cc
@@ -171,8 +171,9 @@ unsigned ExecutionContext::ContextLifecycleStateObserverCountForTesting()
 }
 
 bool ExecutionContext::SharedArrayBufferTransferAllowed() const {
-  if (RuntimeEnabledFeatures::SharedArrayBufferEnabled(this) ||
-      CrossOriginIsolatedCapability()) {
+  if (RuntimeEnabledFeatures::SharedArrayBufferEnabled(this)) {
+  //if (RuntimeEnabledFeatures::SharedArrayBufferEnabled(this) ||
+  //    CrossOriginIsolatedCapability()) {
     return true;
   }
 #if defined(OS_ANDROID)

so that we can continue to test the flag regardless of the isolation headers.

[fmarier]

So the bottom line is that SharedArrayBuffers are already enabled in Brave and so my PR is essentially just a NOOP cleaning up dead code.