[SharedArrayBuffer] Set V8 flags for Atomics, SABs, and Threads

- Reworks code that sets V8 flags for SharedArrayBuffer and Wasm
  Threads to also set the new V8 Atomics flag.
- Atomics are now always enabled.
- SharedArrayBuffer is enabled on desktop, and on Android only with
  site isolation.

Bug: chromium:923807
Change-Id: If40bcbf66216680f0f52ec1bf02dce475b38b1f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343566
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811533}

content/renderer/render_process_impl.cc

@@ -36,6 +36,7 @@
 #include "content/public/common/content_features.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/renderer/content_renderer_client.h"
+#include "services/network/public/cpp/features.h"
 #include "services/service_manager/embedder/switches.h"
 #include "third_party/blink/public/common/features.h"
 #include "third_party/blink/public/web/web_frame.h"
@@ -146,15 +147,28 @@ RenderProcessImpl::RenderProcessImpl()
   SetV8FlagIfFeature(blink::features::kTopLevelAwait,
                      "--harmony-top-level-await");
 
-  if (base::FeatureList::IsEnabled(features::kWebAssemblyThreads)) {
-    constexpr char kFlags[] =
-        "--harmony-sharedarraybuffer "
-        "--experimental-wasm-threads";
+  constexpr char kAtomicsFlag[] = "--harmony-atomics";
+  v8::V8::SetFlagsFromString(kAtomicsFlag, sizeof(kAtomicsFlag));
 
-    v8::V8::SetFlagsFromString(kFlags, sizeof(kFlags));
+  // SharedArrayBuffers require the feature flag, or site isolation. On Android,
+  // the feature is disabled by default, so site isolation is required. On
+  // desktop, site isolation is optional while we migrate existing apps to use
+  // COOP+COEP.
+  bool enableSharedArrayBuffer = false;
+  if (base::FeatureList::IsEnabled(features::kWebAssemblyThreads)) {
+    constexpr char kWasmThreadsFlag[] = "--experimental-wasm-threads";
+    v8::V8::SetFlagsFromString(kWasmThreadsFlag, sizeof(kWasmThreadsFlag));
+    enableSharedArrayBuffer = true;
   } else {
+    enableSharedArrayBuffer =
+        base::FeatureList::IsEnabled(features::kSharedArrayBuffer) ||
+        base::FeatureList::IsEnabled(network::features::kCrossOriginIsolated);
+  }
+
+  if (enableSharedArrayBuffer) {
     SetV8FlagIfFeature(features::kSharedArrayBuffer,
                        "--harmony-sharedarraybuffer");
+  } else {
     SetV8FlagIfNotFeature(features::kSharedArrayBuffer,
                           "--no-harmony-sharedarraybuffer");
   }