[Android WebView] make spellcheck off by default and opt-in using spellcheck="true" attribute.

android_webview/native/aw_settings.cc

Index: android_webview/native/aw_settings.cc
diff --git a/android_webview/native/aw_settings.cc b/android_webview/native/aw_settings.cc
index 9bc886324bcf8164212f9297d6c13bf841e6bf33..3cb71f3941025310049341824a66f55a1930f68d 100644
--- a/android_webview/native/aw_settings.cc
+++ b/android_webview/native/aw_settings.cc
@@ -438,6 +438,11 @@ void AwSettings::PopulateWebPreferencesLocked(JNIEnv* env,
 
   // We use system scrollbars, so make Blink's scrollbars invisible.
   web_prefs->hide_scrollbars = true;
+
+  // Keep spellcheck disabled on html elements unless the spellcheck="true"
+  // attribute is explicitly specified. This "opt-in" behavior is for backward
+  // consistency in apps that use WebView (see crbug.com/652314).
+  web_prefs->spellcheck_enabled_by_default = false;
 }
 
 static jlong Init(JNIEnv* env,