view-manager: Create the root-view only when the viewport-metrics are known.
Instead of creating the root-view at first, delay the creation until the viewport
metrics are known. This avoids having to create the root-view with a default size,
and then updating the size when the viewport metrics become known.
With this change, a Browser knows about the device-pixel ratio as soon as it is
embedded. So there's no more need for DevicePixelRatioWaiter. So that is also
removed.
BUG=496935
Review URL: https://codereview.chromium.org/
1229893004
Cr-Commit-Position: refs/heads/master@{#338905}