How to reproduce:
1. Open the attached project ("case_1146530-CachingClearCacheBug.zip")
2. Make sure that build target is set to WebGL
3. Build and Run the project
4. After the build loads, open developers console and press the "ClearCache" button in the scene

Expected results: Cache gets cleared, build continues to run
Actual results: Build hangs when trying to clear the Cache

Reproducible with: 2019.1.0f2, 2019.2.0a12
Couldn't test with 2017.4, 2018.3 and up to 2019.1.0b1 because of script/build errors

Notes:
1. For successful reproduction, browser with multithread support is required.
2. Tested with Chrome Version (73.0.3683.103 (Official Build) (64-bit)