Auto merge of #3298 - mattwoodrow:swap-chain-memory, r=gw3583
Auto merge of #3298 - mattwoodrow:swap-chain-memory, r=gw3583

Adding swap chain memory field to MemoryReport

Gecko bug 1506492.

This just adds a field for swap chain memory to the report type, we'll fill it in from Gecko.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/webrender/3298)
<!-- Reviewable:end -->

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender