android envsetup: Remove --host-os flag.

Clients should set the host_os gyp variable instead. (As far as I can tell,
this flag was added for the webview build, and this CL updates gyp_webview
to use the gyp define instead. I haven't found other clients.)

Since several gyp targets check the value of host_os in a
target_conditions block and most of the build/common.gypi variables
aren't available during target_conditions processing time, use the same
trick that chromium_code uses to make host_os available at target_conditions
time.

BUG=330631
NOTRY=true

Review URL: https://codereview.chromium.org/153623011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251138 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251138
Add new prerender origin type for <link rel=next ...> launched prerenders.

By adding rel=next as an origin, we get histogram reporting on many of
our existing histograms.

The one tricky decision is that a prerender can have multiple rel
types (both next and prerender), and that by origin we currently
distinguish samedomain vs crossdomain.

In this CL we treat a prerender with both next and prerender rel types
as rel=prerender, and we do not distinguish cross vs same domain on
rel=next prerenders.

R=tburkard,davidben,isherman@chromium.org
BUG=329963

Review URL: https://codereview.chromium.org/121293002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251139 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251139