The glmark2 benchmark score went down (on Android with Pixel devices)
when ANGLE changed VkSwapchainCreateInfoKHR::minImageCount from 3 down
to 2, when using VK_PRESENT_MODE_MAILBOX_KHR.  In experimenting, this
was confirmed.  This CL changes minImageCount back to 3.

Vulkan spec issue #1671 was filed in order to understand whether this
is a good/bad approach for other drivers/platforms.

Test: glmark2 on Pixel2.
Bug: angleproject: 3400
Change-Id: Ia4a72733eb648e4f53feeb8833b174d653fa5766
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1610242
Commit-Queue: Ian Elliott <ianelliott@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>