Make proguard keep everything for hamcrest.

Bug: 1023475
Change-Id: I7df21c024c5459d8fc615e5d33ade559dd16e70b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1910863
Commit-Queue: John Budorick <jbudorick@chromium.org>
Commit-Queue: Yun Liu <yliuyliu@google.com>
Auto-Submit: John Budorick <jbudorick@chromium.org>
Reviewed-by: Yun Liu <yliuyliu@google.com>
Cr-Commit-Position: refs/heads/master@{#714375}

third_party/hamcrest/proguard.flags

--keepnames class org.hamcrest.** { *; }
+-keep class org.hamcrest.** { *; }