[Cronet] add base proguard flags

components/cronet/android/proguard.cfg

-# Keep annotations used by chromium to keep members referenced by native code
--keep class org.chromium.base.annotations.*Native*
--keep class org.chromium.base.annotations.JNINamespace
--keepclasseswithmembers class org.chromium.** {
-    @org.chromium.base.annotations.AccessedByNative <fields>;
-}
--keepclasseswithmembers class org.chromium.** {
-    @org.chromium.base.annotations.*Native* <methods>;
-}
-
-# TODO(mef) remove unnecessary classes from base, so we don't have to preserve
-# their methods
--keepclasseswithmembers class org.chromium.** {
-    native <methods>;
-}
-
-# TODO(xunjieli): Find an alternative to explictly preserving public classes.
--keep public class org.chromium.net.*
-
 -keep class org.chromium.net.impl.CronetUrlRequest$HeadersList
 -keep class org.chromium.net.impl.ChromiumUrlRequest$ResponseHeadersMap
 
-# Needed so that multiple optimization passes will detect annotations
--keepattributes *Annotation*
-
-# Keep methods used by reflection
--keep class org.chromium.base.annotations.UsedByReflection
--keep @org.chromium.base.annotations.UsedByReflection class *
--keepclassmembers class * {
-    @org.chromium.base.annotations.UsedByReflection *;
-}
-
 # Suppress unnecessary warnings.
 -dontnote org.chromium.net.ProxyChangeListener$ProxyReceiver
 -dontnote org.chromium.net.AndroidKeyStore
 # Needs 'void setTextAppearance(int)' (API level 23).
 -dontwarn org.chromium.base.ApiCompatibilityUtils
 # Needs 'boolean onSearchRequested(android.view.SearchEvent)' (API level 23).
 -dontwarn org.chromium.base.WindowCallbackWrapper
 # Generated for chrome apk and not included into cronet.
 -dontwarn org.chromium.base.library_loader.NativeLibraries
 # Generated for chrome apk and not included into cronet.
 -dontwarn org.chromium.base.multidex.ChromiumMultiDexInstaller
 # Objects of this type are passed around by native code, but the class
 # is never used directly by native code. Since the class is not loaded, it does
 # not need to be preserved as an entry point.
 -dontnote org.chromium.net.UrlRequest$ResponseHeadersMap
 # https://android.googlesource.com/platform/sdk/+/marshmallow-mr1-release/files/proguard-android.txt#54
 -dontwarn android.support.**