Steps to reproduce:

1) Download attached project and open in Unity
2) Open scene "SampleScene"
3) Select Main Camera gameObject in the Hierarchy window and make sure that PostProcessLayer script is attached
4) Go to Player Settings and make sure Graphics API is set to Gles2
5) Build and run "SampleScene" on a device
Observe black screen

Expected result: No black screen
Actual result: Using Gles2 Graphics API and PostProcessLayer script gives black screen on a device

Reproduced with:
2018.3.0a1, 2018.3.0a2

Not reproduced with:
2018.1.6f1, 2018.2.0b8

Regression since:
2018.3.0a1

Devices under testing:
VLNQA00006 Samsung Galaxy S7 (SM G930F), OS:7.0, CPU:arm64-v8a, GPU:Mali-T880
VLNQA00052 Xiaomi MI MAX, OS:7.0, CPU:arm64-v8a, GPU:Adreno (TM) 510
VLNQA00134 Samsung Galaxy S8 (SM G950F), OS:7.0, CPU:arm64-v8a, GPU:Mali-G71
VLNQA00003 Razer Phone, OS:7.1.1, CPU:arm64-v8a, GPU:Adreno (TM) 540
VLNQA00139 Vivo Vivo Xplay6*, OS:7.1.1, CPU:arm64-v8a, GPU:Adreno (TM) 530

Note: Works fine using Gles3 or Vulkan Graphics APIs

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Fixed in: 2018.3.0a6