Revert "Android: Upgrade ProGuard 5.2.0 -> 6.0.1"

This reverts commit a1f4e6cb67023ae86fcfe2fc1f8ea5c579994205.

Reason for revert:

This is causing all cronet build failing on cronet_test_instrumentation_apk

Original change's description:
> Android: Upgrade ProGuard 5.2.0 -> 6.0.1
> 
> Shrinks ChromePublic.apk's classes.dex uncompressed by 110kb
> 
> Release notes:
> https://www.guardsquare.com/en/blog/proguard-60-released
> 
> Change-Id: I237dfb7fb35c33671766c217f838ff559d0a4baf
> Reviewed-on: https://chromium-review.googlesource.com/982011
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: agrieve <agrieve@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#546711}

TBR=agrieve@chromium.org,jbudorick@chromium.org

Change-Id: Icf9e21a9c91f6e6de56ec9590514edec2d6c4758
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/986616
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#546918}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5115f94a8bca025a5a2f6a8a15d242d4d88047a1
3 files changed
tree: e026022b0e2b074fe46644f0c99f09773fd33a43
  1. lib/
  2. BUILD.gn
  3. LICENSE
  4. OWNERS
  5. README.chromium