Add a sanitizer.gni import to the GN build.
Add a sanitizer.gni import to the GN build.

The is_*san build flags are moving to this .gni file. This patch will allow
libyuv to continue building when this move happens.

R=brettw@chromium.org, harryjin@google.com
BUG=none

Review URL: https://codereview.chromium.org/1314833005 .

yarrcc/libyuv-ios,disigma/yuv,yarrcc/libyuv-ios,disigma/yuv,lemenkov/libyuv,yarrcc/libyuv-ios,yarrcc/libyuv-ios,lemenkov/libyuv,disigma/yuv,lemenkov/libyuv,lemenkov/libyuv