<chromium> FIXUP Convert sync points to GL fence syncs.

GpuCommandBufferStub::OnRetireSyncPoint is also called as
a result of shutdown destruction where there is a race
condition between the ramp-down of the GLContext and the
DestroyGLFence() and GLFence::CreateWithoutFlush() calls.
This would result in sporadic shutdown crashes or asserts
when a page is closed while browsing WebGL content like
fishgl.com.
Avoid the additional calls if there is no current context.

Task-number: QTBUG-42295
Change-Id: I5aed0df7adca9c95eda71925399d39fd770fffa1
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>