Implement support of unary operator "+" in translator.

BUG=angle:779
TEST=conformance/glsl/misc/struct-unary-operators.html

Change-Id: Ia827e07dcfc8ad3bbbc078e54336815be9027945
Reviewed-on: https://chromium-review.googlesource.com/222720
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>