The description for glFenceSync states:
  glFenceSync — create a new sync object and insert it into the GL
  command stream

ANGLE needs to break the render pass and flush any prior commands to
ensure that the newly created Fence object lands with the correct
ordering. This will be done by calling
ContextVk::flushCommandsAndEndRenderPass() before creating the Vulkan
Event object.

Bug: angleproject:5306
Change-Id: I3a460bb559acf8eb4b3041065ec743938d44d823
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2582199Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>