Fix LESS file dependency tracking in ResourceLoader

Fix LESS file dependency tracking in ResourceLoader

As Timo noted in a review of I1ff6115, using '+=' to combine file dependencies
is incorrect here. I fixed it in one place but not the other, causing the file
dependencies to vary based on whether a .lesscache file was used.

Change-Id: Ie3f48048febc32c5c2d45acd87e10f946f6dfc94