Remove the future serial

This adds 'HasScheduledCommands()', with which Dawn no longer needs the
future serial to tick and track the async tasks.

Bug: dawn:1413
Change-Id: Ide9ba69b796a46fa8bb70b002f4e2aeb1622bffd
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/98720
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>

src/dawn/native/opengl/DeviceGL.cpp

@@ -446,6 +446,12 @@ MaybeError Device::WaitForIdleForDestruction() {
     return {};
 }
 
+bool Device::HasPendingCommands() const {
+    // Technically we could have scheduled commands inside the GL driver that are waiting for a
+    // glFlush but we can't know for sure so we might as well pretend there are no commands.
+    return false;
+}
+
 uint32_t Device::GetOptimalBytesPerRowAlignment() const {
     return 1;
 }

src/dawn/native/opengl/DeviceGL.h

@@ -132,6 +132,7 @@ class Device final : public DeviceBase {
     ResultOrError<ExecutionSerial> CheckAndUpdateCompletedSerials() override;
     void DestroyImpl() override;
     MaybeError WaitForIdleForDestruction() override;
+    bool HasPendingCommands() const override;
 
     const OpenGLFunctions mGL;