[Telemetry] Move page quiescence test to WebContents

Move quiescence test used by speed index so we can more easily reuse it in other parts of the code.

BUG=339180

Review URL: https://codereview.chromium.org/185413005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255851 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255851
Fix typo in ExtensionHost

A backwards conditional could, in theory, result in dev tools not being able
to attach to extension background pages. I cannot reproduce a problem with
this but the code is clearly wrong.

BUG=none
TEST=none (QA - nothing to do here)

Review URL: https://codereview.chromium.org/191173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255852 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255852