Mojom: Blanket change to mark all pointer/handle fields as nullable.
The purpose is to make those APIs have the same semantics as before. And
then we can selectively turn fields into non-nullable.
Mojom files that are not included in this change:
- surface-related files: the owner (jamesr) has landed some non-nullable
fixes. It is possible that he has fully reviewed them. (I will confirm
with him.)
- test files: have been examined.
BUG=324170
TEST=None
Review URL: https://codereview.chromium.org/
502853002
Cr-Commit-Position: refs/heads/master@{#291862}