Android: Disable obfuscation for org.chromium.content.browser.input

Fixes on-screen keyboard becoming unresponsive when you tap on an
<input> element.

Bug: 824048
Change-Id: I28d58cfb2f8aa70461961a49c7ff80ce0a8c1074
Reviewed-on: https://chromium-review.googlesource.com/974082Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544837}

chrome/android/java/proguard.flags

@@ -26,3 +26,9 @@
 -keepclassmembers class android.support.design.internal.BottomNavigationMenuView {
     boolean mShiftingMode;
 }
+
+# There is stack-walking code in our IME that breaks with obfuscation.
+# https://crbug.com/824048.
+-keepnames class org.chromium.content.browser.input.* {
+  *;
+}