[Tizen] Reword Crosswalk's summary in the spec file.

rpmlint currently complains about Crosswalk's Summary entry in its spec
file:

  W: name-repeated-in-summary C Crosswalk
  The name of the package is repeated in its summary. This is often
  redundant information and looks silly in various programs' output.
  Make the summary brief and to the point without including redundant
  information in it.

Follow the suggestion and remove "Crosswalk" from the text.
Merge pull request #2407 from rakuco/improve-spec-summary

[Tizen] Reword Crosswalk's summary in the spec file.