Summary:
When rendering a large list without a getItemLayout (ie SectionList, where it's hard to compute), it freaks out and attempt to render them all, starving the rendering/layout engine from computing the size (and more accurately figurnge out how few it actually should render.)
Before this change, with <FlatList maxToRenderPerBatch={5} data={objectOfLengthN} />, I essentially saw it doing:
```
{first: 0, last: 0}
{first: 0, last: 5}
{first: 0, last: 10}
....
{first: 0, last: N}
```
(no more hiPri renders since all elements have been rendered)
(actually renders and lays out all N objects, and computes their sizes)
(realizes that it doesn't need to show all N for my current screen size and row size)
```{first: 0, last: 55}```
After this change, that whole first part where it keeps incrementing `last` to try to "keep up with the scrolling" disappears.
Closes https://github.com/facebook/react-native/pull/14563
Differential Revision: D5278796
Pulled By: sahrens
fbshipit-source-id: 5db117b40909ec4bc92fba9b5556c6a2add046ac
ebcf5fd241