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We wish to make MostLikely URL recommendations use local thumbnail. To increase the likelihood of a hit, this CL enables a thumbnail request to perform URL prefix match. For example, if thumbnail is stored for
==> http://www.chromium.org/Home
but not
==>  http://www.chromium.org/
then previously the request
==>  chrome://thumb/http://www.chromium.org/
would fail. This CL aims to create a fallback, by adding "thumb2" that matches prefix, i.e.:
==>  chrome://thumb2/http://www.chromium.org/
would match "http://www.chromium.org/Home" and display the corresponding thumbnail. Details:

- We consider "URL prefix" match, which is not same as "string prefix" match. Specifics:
--- Need identical protocol/scheme, host, and port (GURL is smart about this).
--- Query strings are ignored.
--- For path, we require entire path components to match. E.g., "base/test" is a prefix of "base/test/sub", but is NOT a prefix of "base/testing" or "best/test-case".

- If multiple matches exist, the first URL-lexicographical match is taken. This allows the "nearest" children to be matched, but favors siblings that are alphabetically closer. For example, "base" prefers "base/test" over "base/test/sub", but also prefers "base/deep/sub/dir/" over "base/test" (since "deep" < "test").

- To implement the above match, a new comparator is used in the thumbnail cache.

- Adding new test TopSitesCacheTest, which also tests existing code.

- Adding url_utils.cc in chrome\browser\history, along with unit tests.

- Adding the "chrome://thumb2" path, which causes most of the 1-2-line changes in files.

BUG=284634
TEST=TopSitesCacheTest.*

Review URL: https://chromiumcodereview.appspot.com/23477033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@223508 0039d316-1c4b-4281-b951-d872f2087c98