ozone: Fix events_unittests build with use_xkbcommon==1
In r311421 a test was modified that called into
XkbKeyboardLayoutEngine::ParseLayoutName but this function was not exposed in
the header file. This change exposes this method in the header file and fixes
the build.
Original CL that introduced issue: https://codereview.chromium.org/
848833002
BUG=None
TEST=Compile of events_unittests completes
Review URL: https://codereview.chromium.org/
812613004
Cr-Commit-Position: refs/heads/master@{#311523}