Indexed DB: Simplify cursor iteration logic

Indexed DB: Simplify cursor iteration logic

The internal logic for cursor iteration in Continue()
needs to handle four cases: "next", "nextunique", "prev",
"prevunique". This was handled in a single function with a
single loop and hairy `if (forwards) {} else {}` and `if
(unique) {}` clauses, which made understanding and
extending it difficult.

This CL splits the function into two cases: ContinueNext()
for "next"/"nextunique" cursors and ContinuePrevious() for
"prev"/"prevunique" cursors, which greatly simplifies the
logic for each.

The "prevunique" case is particularly complex, since the
spec requires that the first duplicate (in index order) is
returned rather than the last (the one encountered first
when iterating backwards). This was previously done by
iterating further then reversing, all within the same
loop. Now, this case is handled by a subsequent loop.

BUG=497454
R=cmumford@chromium.org

Review URL: https://codereview.chromium.org/1170833004

Cr-Commit-Position: refs/heads/master@{#333780}