Crosswalk embedded support

per request of @ilkkasyrjala I'm pushing the following changes for review, which gives embedded support for Crosswalk on Linux. PTAL @ds-hwang @pozdnyakov. (CC-ing @baleboy @rakuco @poussa @tmpsantos)
`default_size` is not needed to be declared.
```
if (size.IsEmpty())
  size = gfx::Size(kDefaultTestWindowWidthDip, kDefaultTestWindowHeightDip);
```