Bug 1665289 - Convert WebGLChild's kDefaultCmdsShmemSize to a pref. r=lsalzman

Also reduce it from 100KB to 10KB, since we want more concurrency.

Differential Revision: https://phabricator.services.mozilla.com/D90358

dom/canvas/WebGLChild.cpp

@@ -6,12 +6,15 @@
 #include "WebGLChild.h"
 
 #include "ClientWebGLContext.h"
+#include "mozilla/StaticPrefs_webgl.h"
 #include "WebGLMethodDispatcher.h"
 
 namespace mozilla {
 namespace dom {
 
-WebGLChild::WebGLChild(ClientWebGLContext& context) : mContext(&context) {}
+WebGLChild::WebGLChild(ClientWebGLContext& context)
+    : mContext(&context),
+      mDefaultCmdsShmemSize(StaticPrefs::webgl_out_of_process_shmem_size()) {}
 
 WebGLChild::~WebGLChild() { (void)Send__delete__(this); }
 
@@ -21,11 +24,9 @@ void WebGLChild::ActorDestroy(ActorDestroyReason why) {
 
 // -
 
-static constexpr size_t kDefaultCmdsShmemSize = 100 * 1000;
-
 Maybe<Range<uint8_t>> WebGLChild::AllocPendingCmdBytes(const size_t size) {
   if (!mPendingCmdsShmem) {
-    size_t capacity = kDefaultCmdsShmemSize;
+    size_t capacity = mDefaultCmdsShmemSize;
     if (capacity < size) {
       capacity = size;
     }

dom/canvas/WebGLChild.h

@@ -34,6 +34,7 @@ struct FlushedCmdInfo final {
 
 class WebGLChild final : public PWebGLChild, public SupportsWeakPtr {
   const WeakPtr<ClientWebGLContext> mContext;
+  const size_t mDefaultCmdsShmemSize;
   webgl::RaiiShmem mPendingCmdsShmem;
   size_t mPendingCmdsPos = 0;
   FlushedCmdInfo mFlushedCmdInfo;

modules/libpref/init/StaticPrefList.yaml

@@ -9937,6 +9937,11 @@
   value: false
   mirror: always
 
+- name: webgl.out-of-process.shmem-size
+  type: RelaxedAtomicUint32
+  value: 10000 # 10KB
+  mirror: always
+
 - name: webgl.power-preference-override
   type: RelaxedAtomicInt32
   value: 0