How to reproduce:
1. Open the "WebRequest_Leak.zip" project
2. Open the "SampleScene.unity" scene
3. Enter Play Mode
4. Press the "Button" button
5. Check Unity's memory usage

Actual result: Unity app memory usage is slowly rising. (Screenshot attached)
Expected result: Unity app memory usage is not rising.

Reproducible with: 2020.1.0a1, 2019.3.0a12, 2019.2.2f1, 2018.4.7f1, 2017.4.32f1.

Devices reproducible with:
- VLNQA00272, Samsung Galaxy S10+ (SM-G975U), Android 9, CPU: Snapdragon 855 SM8150, GPU: Adreno (TM) 640
- iPhone 7 (iOS 10.2)

Notes:
- Reproducible on iOS, Android and Editor.
- Not reproducible on Mac Standalone.
- It's easier to notice the leak on iOS.

----------------------------
Fixed in 2020.1.0a6, 2019.3.0b5, 2019.2.8f1.