Fix typo in //third_party/junit/proguard.flags

-keepname -> -keepnames

Change-Id: Ie8febb9c4239e5344b0e3e6b333b4576cccb84b3
Reviewed-on: https://chromium-review.googlesource.com/1185692Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585695}

third_party/junit/proguard.flags

@@ -3,6 +3,6 @@
 -dontwarn org.junit.**
 -dontwarn org.hamcrest.**
 
--keepname,allowoptimization public class junit.framework.** {
+-keepnames,allowoptimization public class junit.framework.** {
     *;
 }