Apk for test proguard flags ignoring classes which don't exist

Bug: 872904
Change-Id: Ib178be906c8d74ecabd7cfafeba8e7cf3a3398ab
Reviewed-on: https://chromium-review.googlesource.com/c/1327331Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606836}

chrome/android/java/apk_for_test.flags

@@ -9,5 +9,10 @@
 # -keep for **.test.**.
 -keepnames class **.test.** { *; }
 
+# These are Java classes which aren't part of the Android sdk. They are
+# referenced by some test-only libraries.
+-dontwarn java.lang.ClassValue
+-dontwarn java.lang.instrument.ClassFileTransformer
+
 # Do not mark classes as final, as doing so may interfere with mocking.
 -optimizations !class/marking/final