Complete the removal of ProcessMetrics::CalculateFreeMemory

r281335 left the header definition.

BUG=389703

Review URL: https://codereview.chromium.org/421143002

git-svn-id: https://src.chromium.org/svn/trunk/src/base@285869 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
MSan: increase the test timeout multiplier to 6x.

BUG=178409
TBR=phajdan.jr@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/424693005

git-svn-id: https://src.chromium.org/svn/trunk/src/base@285891 4ff67af0-8c30-449e-8e8b-ad334ec8d88c