Relax strict restriction on position of line-continuation

This fixes comments#19 in https://codereview.chromium.org/2082483002/.

BUG=295792
TEST=deqp/data/gles3/shaders/preprocessor.html
CQ_INCLUDE_TRYBOTS=tryserver.chromium.win:win_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2129883002
Cr-Commit-Position: refs/heads/master@{#404335}
1 file changed