direct_memory_pressure_calculator.h: remove 'virtual'

I messed this up in https://codereview.chromium.org/1314553003

BUG=82385, 520962
TBR=chrisha

Review URL: https://codereview.chromium.org/1312893002 .

Cr-Commit-Position: refs/heads/master@{#345152}
1 file changed