Reduce the AppCache maximum size to a more reasonable value in DumpRenderTree2

Without this change, any use of AppCache fails after the test
http/tests/appcache/max-size.html has been run. The test in question
passes, but seems to leave the database in a corrupted state, causing
all future database transactions to fail. The only fix is to remove
the database file.

This behaviour is only observed with DumpRenderTree2, not
DumpRenderTree, so I don't think this is a WebView problem. I'm not
sure what the root cause is, but I think it make sense to submit this
fix now in order to get the LayoutTests stable with DumpRenderTree2.

Change-Id: I763c09406e66839676b839e3cdeba3b9427c1cb9