Steps to repro:

1. Open attached project;
2. In Project folder go to Assets -> TerrainDestruction -> Demo;
3. Open 'TerrainDeformer' scene;
4. Build&Run scene for WebGl;
5. In the opened project click on terrain to modify its heightmap

Expected result:
Terrain heightmap is lowered down with craters created on its surface. See attached terrainRaycastExpected gif video.

Actual result:
Terrain heightmap is no modified. See attached terrainRaycastActual gif video.

Notes:
- Reproducible on Google Chrome, Version 56.0.2924.87 (64-bit);
- Reproducible in 5.6.0b9, 5.5.0p1;
- Reproducible on both OSX 10.12.2 and Windows 10;
- Reproducible only in WebGl, not reproducible in Editor and in Gl/DX Standalone.