How to reproduce:
1. Open the "SampleScene" from the attached project "Timeline.zip"
2. Press play and notice that the cube is moving
3. Build the project for iOS platform and notice the cube isn't moving anymore

Expected result: When Using Managed Stripping Level of Medium or High, Timeline/PlayableDirector should work in builds for iOS as it does for Android
Actual result: Timeline/PlayableDirector stops working in builds for iOS when using Managed Stripping Level of Medium or High

[Android] IL2CPP and Mono

Tested with these versions: 2018.3.14f1, 2019.1.0f2, 2019.2.0a13
On these devices:
VLNQA00002, Samsung Galaxy S5 Neo (SM-G903F), Android 6.0.1, CPU: Exynos 7 Octa 7580, GPU: Mali-T720
VLNQA00230, Oneplus OnePlus6T (ONEPLUS A6013), Android 9, CPU: Snapdragon 845 SDM845, GPU: Adreno (TM) 630
VLNQA00173, Xiaomi Redmi 6 Pro (Redmi 6 Pro), Android 8.1.0, CPU: Snapdragon 625 MSM8953, GPU: Adreno (TM) 506

[iOS]
VLNQA00246, iPhone 6 Plus iOS 12.1.4 (16D57), 64GB, Model MGAH2B/A
VLNQA00232, iPhone 8 Plus, iOS 12.1.2 (16C101), 64GB, ModelMQ8M2ET/A
HELA0167, iPad 6th gen iOS 11.4.1 (15G77), 32GB, Model MR7F2KN/A

Regression introduced in:
2018.3.0a2

Crashes in Xcode with:
2018.3.0a10, 2018.3.0a11, 2018.3.0b1,
2019.1.0b4

Reproducible with:
2018.3.b08, 2018.3.0f1, 2018.3.7f1, 2018.3.14f1,
2019.1.0b5, 2019.1.1f1
2019.2.0a13

Couldn't reproduce due to errors in the editor:
Build completed with a result of 'Failed'
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
2018.2.20f1, 2018.2.21f1
2018.3.0a5, 2018.3.0a6, 2018.3.0a7, 2018.3.0a8, 2018.3.0a9
2019.1.0a1, 2019.1.0a2, 2019.1.0a5, 2019.1.0a13, 2019.1.0a14, 2019.1.0b3

Note: In order to work around it, you need to switch Managed Stripping Level to Low