Sync gyp_xwalk with the current contents of gyp_chromium.

gyp_chromium underwent a large amount of changes since M32 due to the work on
GN upstream. This broke the Android build (gyp_xwalk did not even detect it
was supposed to set up an Android build in the first place).

I have basically copied gyp_chromium on top of gyp_xwalk and applied the 
relevant changes after comparing gyp_xwalk with M32's gyp_chromium, with the 
addition of a change that does not run GN when using the make generator (as a
temporary measure to keep the Tizen build working while we find a better 
solution).