Part 3 of search provider refactoring.

Moves static methods used by both search_provider and
zero_suggest_provider into the base class. Does not change any
implementation details. This allows us to remove
friendship between zero_suggest_provider and base_search_provider
classes.

TBR=mpearson

BUG=338955

Review URL: https://codereview.chromium.org/149243007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249840 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249840
Clarify event page documentation for apps vs extensions.

Apps specify "background" nested in an "app" dictionary, where
extensions it is just a top level entry.

BUG=340406
NOTRY=true

Review URL: https://codereview.chromium.org/157323003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249841 0039d316-1c4b-4281-b951-d872f2087c98
Disable flaky unittest

TracingControllerTest.EnableCaptureAndDisableMonitoringWithFilePath has been flaking consistently.

NOTRY=true
TBR=wangxianzhu
BUG=341995

Review URL: https://codereview.chromium.org/157923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249842 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249842