Fix Cronet compilation and test errors.

components/cronet/android/proguard.cfg

 # Keep annotations used by chromium to keep members referenced by native code
 -keep class org.chromium.base.*Native*
 -keep class org.chromium.base.JNINamespace
 -keepclasseswithmembers class org.chromium.** {
     @org.chromium.base.AccessedByNative <fields>;
 }
 -keepclasseswithmembers class org.chromium.** {
     @org.chromium.base.*Native* <methods>;
 }
 
 # TODO(mef) remove unnecessary classes from base, so we don't have to preserve
 # their methods
--keepclasseswithmembers class org.chromium.** {
+-keep class org.chromium.** {
     native <methods>;
 }
 
--dontnote org.chromium.net.AndroidKeyStore
-
 # Needed so that multiple optimization passes will detect annotations
 -keepattributes *Annotation*
 
 # Keep methods used by reflection and native code
 -keep class org.chromium.net.UsedBy*
 -keep @org.chromium.net.UsedBy* class *
 -keepclassmembers class * {
     @org.chromium.net.UsedBy* *;
-}
+}
+
+# Suppress unnecessary warnings.
+-dontnote org.chromium.net.AndroidKeyStore
+# Objects of this type are passed around by native code, but the class
+# is never used directly by native code. Since the class is not loaded, it does
+# not need to be preserved as an entry point.
+-dontnote org.chromium.net.UrlRequest$ResponseHeadersMap
+