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This is a re-land of
https://codereview.chromium.org/471673002/
which was reverted because it caused a failure in
interactive_ui_tests
InstantExtendedPrefetchTest.SetPrefetchQuery (run #1):
[ RUN      ] InstantExtendedPrefetchTest.SetPrefetchQuery
../../chrome/browser/ui/search/instant_extended_interactive_uitest.cc:867: Failure
Value of: SearchProvider::ShouldPrefetch(*( omnibox()->model()->result().default_match()))
  Actual: false
Expected: true
[  FAILED  ] InstantExtendedPrefetchTest.SetPrefetchQuery, where TypeParam =  and GetParam() =  (1613 ms)

http://build.chromium.org/p/chromium.linux/buildstatus?builder=Linux%20Tests&number=12979

It has two changes:
- revises the failing tests
- does some requested cleanup of the unit tests

The original changelist description follows:
---

Calls to the suggest server may normally result in a new inline
autocompletion.  This can be disruptive because it means pressing enter
may bring the user to different places depending on how long he/she
waits after typing the last key.

This change prevents new suggestions from becoming the default match.
In other words, the default match is only allowed to change on a
keystroke, not due to a reply coming back from the server.

The consequence of this change is that if previously we'd show an
inline suggestion on a server reply, now we only show it one keystroke
later.  I think this trade-off (one keystroke versus inconsistent
omnibox behavior) is a good one to make.

We still end up with default matches (inline autocompletions within
the omnibox) from the suggest server after this change.  Here's an
example of why:

User types "facebo"
We send a suggest server request.
Server asynchronously returns "facebook" as a top suggestion,
  beating the server-provided verbatim score for "facebo".
We decide not to show it within the omnibox.  It's instead shown
  somewhere in the dropdown.
User types "o".
We send a suggest server request.
We reuse our cached suggestions and suggestion scores.  <<< THE KEY
We show "facebook" as an inline suggestion because it beats
  the default verbatim score that gets assigned to "faceboo".
  (This is the score that we assign by default without having
  yet received the most recent suggest server response.)
We receive the response, which includes "facebook" as a top
  suggestion, beating the server-provided verbatim score
  for "faceboo".
We show "facebook" as an inline suggestion.  i.e., we decide
  not to demote it because it was already being shown inline

TESTED:
unit tests plus interactive tests (facebook.com/l, google.com/a)

BUG=398135

Review URL: https://codereview.chromium.org/481693004

Cr-Commit-Position: refs/heads/master@{#293049}