Eagerly destroy CommandBuffer commands after submission

Command buffers hold references to all encoded objects. Freeing them
eagerly significantly reduces the amount memory held before the JS GC
clears the command buffers.

Bug: dawn:262, dawn:372
Change-Id: I68dfa973f980fba8d94611ed1de3c593bdb91a63
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/26562
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn_native/CommandAllocator.h

@@ -91,10 +91,13 @@ namespace dawn_native {
             return static_cast<T*>(NextData(sizeof(T) * count, alignof(T)));
         }
 
-        // Needs to be called if iteration was stopped early.
+        // Sets iterator to the beginning of the commands without emptying the list. This method can
+        // be used if iteration was stopped early and the iterator needs to be restarted.
         void Reset();
 
-        void DataWasDestroyed();
+        // This method must to be called after commands have been deleted. This indicates that the
+        // commands have been submitted and they are no longer valid.
+        void MakeEmptyAsDataWasDestroyed();
 
       private:
         bool IsEmpty() const;
@@ -139,7 +142,6 @@ namespace dawn_native {
         size_t mCurrentBlock = 0;
         // Used to avoid a special case for empty iterators.
         uint32_t mEndOfBlock = detail::kEndOfBlock;
-        bool mDataWasDestroyed = false;
     };
 
     class CommandAllocator {