[Mono.Android] abstract-ify inconsistent android.webkit API.

As part of discussion at #1078 (comment) ,
we decided to make changes to those inconsistent methods that used to be
virtual and then became abstract, to become abstract even in old API level.

It is done at metadata fixup level.

Author: atsushieno

src/Mono.Android/metadata

@@ -1,4 +1,16 @@
 <metadata>
+
+  <!-- FIXME: these members used to be `virtual` and then became `abstract`.
+  They result in API breakage (reported as inter-api-level mismatch).
+  This is ancient Google's fault. To deal with the situation nicely,
+  we are marking them just as `abstract` even at the old API Level, so
+  that app developers don't get trapped due to missing members at run time.
+  -->
+  <attr path="/api/package[@name='android.webkit']/class[@name='CookieManager' or @name='WebBackForwardList' or @name='WebHistoryItem' or @name='WebIconDatabase' or @name='WebViewDatabase']/method[@static='false']" name="abstract">true</attr>
+  <attr path="/api/package[@name='android.webkit']/class[@name='WebSettings']/method[@static='false' and @name!='setTextSize']" name="abstract">true</attr>
+  <attr path="/api/package[@name='android.webkit']/class[@name='CookieManager' or @name='WebBackForwardList' or @name='WebHistoryItem' or @name='WebIconDatabase' or @name='WebViewDatabase']" name="abstract">true</attr>
+  <attr path="/api/package[@name='android.webkit']/class[@name='WebSettings']" name="abstract">true</attr>
+
   <!-- Manifest.permission has its last name part all in lowercase and
        regarded as obfuscated, so avoid that by explicitly marking not. -->
   <attr path="/api/package[@name='android']/class[@name='Manifest.permission']" name="obfuscated">false</attr>