Summary:
When calculating how far the user has scrolled, it is necessary to know the content insets where:
number of pixels scrolled = content offset + leading content inset for the scroll axis
This diff adds the contentInset field to native scroll events.
Closes https://github.com/facebook/react-native/pull/737
Github Author: James Ide <ide@jameside.com>
Test Plan: Imported from GitHub, without a `Test Plan:` line.