* [Collections] Allow custom UIEdgeInsets
MDCCollectionViewController implements custom logic for computing the
`UIEdgeInsets` depending on the `MDCCollectionViewCellStyle` and the
`MDCCollectionViewCellLayoutType`. Subclasses need to override this
functionality to customize their presentation.
* MDCCollectionViewController will use the public
`-collectionView:layout:section` to compute the width of its
supplementary view instead of the internal implementation directly.
* Update the Collections layout example to have different horizontal
edge insets depending on the view's horizontal size class.
Closes#1507
* Add EarlGrey test to verify the insets exampl
* Removing changes to Collection Cells example
* Running code formatter
* Supplemental files.
* Some code clean-up based on willlarche@'s comments
* Removing old header, small cleanup
* Changing block parameter types to match the desired class
* One last id to remove
* Making Earl Grey test a bit better. Some formatting
* Fixing a bug with the flowLayout delegate