Deferred clears are not collected when syncing the READ framebuffer.
Prior to this change, we had in FramebufferVk::syncState:

    if (READ && deferredClears.any())
    {
        flushDeferredClears();
    }

However, this is impossible / unnecessary:

- Every operation whose syncState (for the DRAW framebuffer) collects
  deferred clears will flush the deferred clears.  In fact, it's an
  error for the next operation's syncState to encounter pre-existing
  deferred clears.
- The READ framebuffer is synced before the DRAW framebuffer.  This
  makes it impossible for there to be deferred clears when READ is
  synced.

It may be necessary to swap the order in which the READ and DRAW
framebuffers are synced.  See http://anglebug.com/5266.  In that case,
if the READ and DRAW framebuffers are identical:

- The DRAW framebuffer's sync will collect deferred clears.  The READ
  framebuffer's sync will see deferred clears, but it must not flush
  them!

Bug: angleproject:4988
Change-Id: I179002d739608ccb8bda95d4379dc6d54e2bf4bb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2511372
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>