Generalize browser support in the infrastructure

[athomas]

• Add browser to the deps with conditions (e.g. "download_chrome", https://dart-review.googlesource.com/c/sdk/+/152340/).
• Add a recipe property that allows builders to specify extra gclient vars to all builders that require browsers (https://dart-review.googlesource.com/c/recipes/+/152380).
• Add neo.py support to handle the new property (https://dart-review.googlesource.com/c/recipes/+/153120).
• Add download_chrome to pkg builders (https://dart-review.googlesource.com/c/sdk/+/152502).
• Use the new properties on all chrome and firefox builders (https://dart-review.googlesource.com/c/sdk/+/153206/).
• Pass browser location to tests via an environment variable (https://dart-review.googlesource.com/c/sdk/+/153209).

[athomas]

This is completed.