Omnibox: Add Histograms to Time Parts of SearchProvider

Hopefully these will help me narrow down what part of SearchProvider
is slow.

I didn't add histograms to parts of the code that are only
executed when suggest is enabled.  People reporting the slowness
attest that the slowness appears while suggest is disabled;
I don't need to monitor the suggest section of the code at all.

BUG=237703,178705,262263,168933

Review URL: https://codereview.chromium.org/23469012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225323 0039d316-1c4b-4281-b951-d872f2087c98