Generate getters/setters for remaining flex-* properties.

This patch changes the following storage_only fields to be primitive
or external fields:
- flex-basis
- flex-grow
- flex-shrink

so that their getters/setters can be generated.

Diff: https://gist.github.com/cdc56f6747da1a925498c52831557fa2/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2943413003
Cr-Commit-Position: refs/heads/master@{#481102}
2 files changed