Build fix linux aura without USE_XI2_MT.

touch_factory_x11.cc:96 must not use XScopedString because it is defined ui/base.
ui/base depends on ui/events.

Cherry-pick from https://codereview.chromium.org/49383011/
Merge pull request #73 from ds-hwang/pinch

Build fix linux aura without USE_XI2_MT.