This was addressed by https://codereview.chromium.org/2508703002/ "VrShell/WebVR content viewport refactoring", landed as https://crrev.com/4026d565687ddbc1f67c4cc04887c339b7a03685
Cr-Commit-Position: refs/heads/master@{#432811} and is included in v56 canary.

The WebVR compositor path is now rendering the content GL canvas at native window resolution, for example 2560x1440 for a Pixel XL. This isn't quite 1:1 pixel mapping post-distortion, but at least it minimizes sampling artifacts since the same resolution is used throughout the rendering pipeline until it's passed to GVR for distortion, and it's fairly similar to the recommended GVR rendering resolution.