Add the check for return value of GetString().

This patch is to add the check for return value of GetString() in
widget_handler.cc and xwalk_content.cc.

CID=194059
CID=220010

Related to XWALK-2928.
Refer to https://w3c.github.io/manifest/#dfn-display-modes-values, the display mode string can be "fullscreen | standalone | minimal-ui | browser".

If I'm correct for the standard, I think we should use casecmp instead of find. We can follow what chromium does, in [content/renderer/manifest/manifest_parser.cc::223](https://code.google.com/p/chromium/codesearch#chromium/src/content/renderer/manifest/manifest_parser.cc&q=content/renderer/manifest/manifest_parser.cc&type=cs&sq=package:chromium&l=223)