Steps to reproduce:
1. Open the attached project (GetPixel32Failed.zip)
2. Open the Assets/TestScene.unity scene
3. Switch platform to WebGL
4. You should notice to the above two errors in the console when loading the scene:
GetPixels32 failed: insufficent pixel buffer size (4), must be at least 4 x 4
Failed to get pixels of splat texture
5. Begin painting any of the textures on the terrain in the scene
6. Note the two errors listed above in the console repeated multiple time

Expected result: Painting terrain without errors
Actual result: Console printing error messages

Note:
These error messages only appear when the platform is set to WebGL.

Reproduced with: 5.3.6p8, 5.4.3p4, 5.5.0f3, 5.5.0p2, 5.6.0b2