GitLab

This CL adds a mechanism to isolate sheet-specific code previously in
PaymentRequestDialog to Controller classes. This will allow for clearer
view creation and event handling as the different sheets in the dialog
are implemented.

BUG=667872

Review-Url: https://codereview.chromium.org/2579513002
Cr-Commit-Position: refs/heads/master@{#439141}