Add proguard flags to make JaCoCo work on release build

Bug: 988116
Change-Id: I046e4e17b4aa1e5b6741123e4585c4c82470815d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742658Reviewed-by: Sam Maier <smaier@chromium.org>
Reviewed-by: Yun Liu <yliuyliu@google.com>
Commit-Queue: Yun Liu <yliuyliu@google.com>
Cr-Commit-Position: refs/heads/master@{#685370}

third_party/jacoco/jacoco_instrument.flags

 # This package is only used when Java coverage is enabled.
 # JaCoCo libraries contain all dependencies so we can ignore warning here.
 -dontwarn java.lang.instrument.**
+
+# Keep everything for the jacoco classes.
+-keep class com.vladium.** {*;}
+-keep class org.jacoco.** {*;}
+-dontwarn org.jacoco.**