ozone: scenic: Initialize device pixel ratio in ScenicWindow

When the view properties event arrives before the view metrics event, we
get a wildly incorrect surface size due to use of an unitialized scale
factor.

We're already checking if it's nonzero before deciding the size, so
initialize it to zero so that this check can work.

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

Change-Id: I6049e05992b1041ee1f63b1f293f1ac2013c82b0

Reviewed-on: https://chromium-review.googlesource.com/c/1279286
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599385}
1 file changed