chromeos: Don't send empty power management policies.

Update PowerPolicyController to not send an empty policy to
powerd when Chrome starts or exits. Empty policies were
originally sent to ensure the system wouldn't be stuck with
an old policy, but they also mean that powerd could briefly
revert to its default settings with undesireable results
when the system first boots or the user logs out (e.g. an
enterprise policy has disabled the lid-closed action, but
powerd reverts to its default settings and shuts down
automatically when the user logs out).

BUG=329930
TBR=benwells@chromium.org

Review URL: https://codereview.chromium.org/116983008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243566 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243566
Break dependency of chrome.gyp:common on content.gyp:content_browser.

The dependency path is
  src/chrome/chrome.gyp:common ->
  src/components/components.gyp:policy_component ->
  src/components/components.gyp:user_prefs ->
  src/content/content.gyp:content_browser

This was introduced in https://codereview.chromium.org/102973005. The issue is
that URLBlacklistManager::RegisterProfilePrefs() uses a PrefServiceSyncable
which comes from user_prefs, and user_prefs depends on content::BrowserContext.

This fix splits the policy/common files into a policy_component_common target
that does not depend on user_prefs. chrome.gyp:common now depends on
policy_component_common, and chrome.gyp:browser depends on policy_component.

BUG=332030

Review URL: https://codereview.chromium.org/124393003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243567 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243567