Update WebView manifest to target/minSDK 21.

Bug: 16902981
Change-Id: I44594b62e94b034662b30f9e43a65946224c3152
diff --git a/chromium/AndroidManifest.xml b/chromium/AndroidManifest.xml
index 8adc119..8d57481 100644
--- a/chromium/AndroidManifest.xml
+++ b/chromium/AndroidManifest.xml
@@ -16,8 +16,8 @@
 
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.android.webview">
-    <uses-sdk android:minSdkVersion="20"
-              android:targetSdkVersion="20">
+    <uses-sdk android:minSdkVersion="21"
+              android:targetSdkVersion="21">
     </uses-sdk>
     <application android:label="Android System WebView"
         android:icon="@drawable/icon_webview"