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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