ozone: demo: Defer renderer creation until nonzero size

Fuchsia platform windows are initially 0x0 and this causes vulkan surface
creation to crash. Defer creating a renderer until we have a nonempty
size.

Bug: 861853
Test: run_ozone_demo --ozone-platform=scenic --enable-vulkan

Change-Id: I74e14ed07ab47ebc22730b40cf36217e9ae4873a
Reviewed-on: https://chromium-review.googlesource.com/1229162
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592110}
2 files changed