Prerequisites:
You'll need the Labs Shadowmask project: https://ono.unity3d.com/test-data/gi-tests/LabsShadowmask

Steps to repro:
1. Open attached project and 'TheLabs_Shadowmask' scene;
2. In the Lighting window select 'Progressive GPU' lightmapper;
3. In the Lighting window click on 'Generate Ligthing' button.

Expected result:
Baking on GPU is executed without falling back to CPU Lightmapper or asserts.

Actual result:
Lightmapper backend falls back to CPU Lightmapper. The fallback is accompanied with the following errors:

'OpenCL Error. Falling back to CPU lightmapper. Error callback from context: CL_PROFILING_INFO_NOT_AVAILABLE'

and

`Assertion failed on expression: 'IsCLEventCompleted(events->m_StartMarker, isStartEventAnError)'`.
See attached video and actual.png screenshot.

Notes:
- Reproducible in 2019.1.0a1, 2018.3.0b3, 2018.3.0b2, 2018.3.0b1. ;
- Not reproducible in 2018.3.0a11;
- Reproducible when baking GeForce GTX 1070 and GeForce 640M; not reproducible on GeForce GTX 1080.
- Reproducible on Windows 10;
- Assert in IsCLEventCompleted might not be directly related to the fallback to CPU LM bug;
- The issue also happens in the new project with cubes and planes;
- The bug might not happen every time; if it doesn't try reloading Editor, cleaning GI cache/baked data and rebaking. Additionally, bake several time with changing light settings (Mixed>Baked) and move the viewport while baking.