Fix double listing of memory_pressure_monitor cc files

Fix copy-paste error in https://codereview.chromium.org/1149223002/.

TBR=jam@chromium.org
BUG=490824, gyp:384, 472772

Review URL: https://codereview.chromium.org/1150233003

Cr-Commit-Position: refs/heads/master@{#331175}
1 file changed