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.
@halton At content/renderer/manifest/manifest_parser.cc::223, the comparision with display mode is case-insensitive, should we follow it instead of use casecmp? Thanks.