Create a compile time flag for Shared Process Model

Before this change Shared Process Model could be enabled
using 'xwalk' command line argument, this however does not
fit now as 'xwalkctl' also needs to be aware whether
Shared Process Model is enabled or not.
The 'shared_process_mode' gyp flag was introduced to solve the
issue described above.
Wrong trailing backslash. It's easier to just add it _above_ `enable_hidpi`.