[Fuchsia] Disable semaphore reuse.

ExternalVkImageSkiaRepresentation uses ExternalSemaphorePool to re-use
semaphores. OutputPresenterFuchsia converts image semaphores to Zircon
events and shares them with Scenic. Scenic waits for these events, but
never resets them. As result the semaphores are often re-used in signaled
state. This CL disables semaphore re-use on Fuchsia to avoid issue.

Bug: 1140719
Change-Id: I9154535b59262d60cd602c7711315673cf2bad85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487879
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Reviewed-by: John Bauman <jbauman@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819425}