Remove outdated comment from common.gypi.

BUG=None
TEST=None

Review URL: https://codereview.chromium.org/196983008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257016 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 257016
Media Galleries API Metadata: Add libexif to list of Linux system library deps

Adds libexif to list of system libs chromium build on Linux depends on. In service of https://codereview.chromium.org/191583002/

BUG=318450

Review URL: https://codereview.chromium.org/197653006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257017 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 257017