Add partial magnifier to ash palette.

The partial magnifier will magnify a small portion of the screen, similar to a spyglass.

TEST=./out/Release/ash_unittests --gtest_filter=PartialMagnificationControllerTest.*
TBR=oshima@chromium.org

BUG=616112

Review-Url: https://codereview.chromium.org/2239553002
Cr-Commit-Position: refs/heads/master@{#414124}
bool CanHandleTouchHud() {
  return RootWindowController::ForTargetRootWindow()->touch_hud_debug();
}

bool CanHandleTouchHud() {
  return RootWindowController::ForTargetRootWindow()->touch_hud_debug();
}
