[Android]Fix a regression that snackbar sits behind the scrim

chrome/android/java/proguard.flags

Index: chrome/android/java/proguard.flags
diff --git a/chrome/android/java/proguard.flags b/chrome/android/java/proguard.flags
index 4f9a785fa91581a72d799c652e99f9acdc4a4f27..3f660279efb1a3b34421f7b701087b1795534e62 100644
--- a/chrome/android/java/proguard.flags
+++ b/chrome/android/java/proguard.flags
@@ -25,6 +25,13 @@
 -keep class org.chromium.content.app.SandboxedProcessService
 -keep class org.chromium.content.app.PrivilegedProcessService
 
+# TODO(ianwen): remove this config once proguard file inclusion from AAR is supported.
+# CoordinatorLayout resolves the behaviors of its child components with reflection.
+-keep public class * extends android.support.design.widget.CoordinatorLayout$Behavior {
+    public <init>(android.content.Context, android.util.AttributeSet);
+    public <init>();
+}
+
 # SearchView is used in website_preferences_menu.xml and is constructed by
 # Android using reflection.
 -keep class android.support.v7.widget.SearchView {