When paddingStart is 0, it should override paddingHorizontal (#816)
When paddingStart is 0, it should override paddingHorizontal (#816)

Summary:
Fixes #815

Imagine a node with this style: `{ paddingHorizontal: 10, paddingStart: 0 }`.

After running layout on this node, we expect its computed `paddingStart` to be `0`. However, it is actually `10`.

Consider the expression `paddingEdgeStart.getValue() > 0.0f` in [`getLeadingPadding`](https://github.com/facebook/yoga/blob/328ec7dc4d104b42b836d5ccebff04033d045133/yoga/YGNode.cpp#L461). Why is `0` handled like a negative number rather than a positive number? I suspect this should be `>=` so `0` is handled like the positive numbers (this is how `getTrailingPadding` works).

It looks like https://github.com/facebook/yoga/commit/3a82d2b1a8f5b65a2c3bd1407552d3ed2226238e?diff=unified&w=1#diff-07b4949bf42749fde386e769ff08a124 changed the operator from `>=` to `>` in `getLeadingPadding`. I suspect it was a mistake. `getTrailingPadding` still uses `>=`.

I manually verified this using the code in #815 and added some unit tests to catch this bug and other similar issues.

Adam Comella
Microsoft Corp.
Pull Request resolved: https://github.com/facebook/yoga/pull/816

Reviewed By: priteshrnandgaonkar

Differential Revision: D10282617

Pulled By: shergin

fbshipit-source-id: 2ab2874ae39d9454308a020a960ace85573fe777

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho