[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}

gpu/command_buffer/service/external_semaphore_pool.cc

@@ -15,11 +15,14 @@
 namespace gpu {
 namespace {
 
-#if defined(OS_ANDROID)
+#if defined(OS_ANDROID) || defined(OS_FUCHSIA)
 // On Android, semaphores are created with handle type
 // VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_SYNC_FD_BIT. With this handle type,
 // the semaphore will not be reset to un-signalled state after waiting,
 // so semaphores cannot be reused on Android.
+// On Fuchsia semaphores are passed to scenic as zx::event. Scenic doesn't reset
+// them after waiting, so they would have to be reset explicitly to be reused.
+// OTOH new semaphores are cheap, so reuse doesn't provide significant benefits.
 constexpr size_t kMaxSemaphoresInPool = 0;
 #else
 constexpr size_t kMaxSemaphoresInPool = 16;