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>