Switch Component Updater to use a blocking thread pool.

This way, we no longer block the FILE thread for multi-second or multi-
minute operations.

BUG=

Review URL: https://codereview.chromium.org/52653003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232065 0039d316-1c4b-4281-b951-d872f2087c98
[chromedriver] Disable perf tests.

BUG=chromedriver:602

Review URL: https://codereview.chromium.org/52753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232066 0039d316-1c4b-4281-b951-d872f2087c98
Fix minor typo in comments

BUG=

Review URL: https://codereview.chromium.org/52793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232067 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232067