IndexedDB: Switched two Iterator::Seek calls to Transaction::Get.

IndexedDB: Switched two Iterator::Seek calls to Transaction::Get.

Switching for two reasons:

1. Semantics are simpler because, in each of these cases, there was never a
   subsequent iteration - just a check to see if the iterator key matched
   the desired key.
2. The Transaction::Get() call uses the LevelDB filter which (when enabled)
   can significantly reduce disk reads, thus improving performance.

This change, along with enabling the bloom filter, resolves a performance
issue introduced by r273048.

BUG=409520
Committed: https://crrev.com/7eff2e4b1a12584cd89f48ca3b9879faa37ebedc
Cr-Commit-Position: refs/heads/master@{#294675}

[cmumford, 2014-09-11 23:51:56]

cmumford@chromium.org changed reviewers:
+ jsbell@chromium.org

[cmumford, 2014-09-11 23:51:56]

I felt this change was good to do even if we don't enable the Bloom filter so
decided to submit in a separate CL.

[jsbell, 2014-09-11 23:56:33]

lgtm

[cmumford, 2014-09-12 20:59:42]

The CQ bit was checked by cmumford@chromium.org

[commit-bot: I haz the power, 2014-09-12 21:00:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/565023002/1

[commit-bot: I haz the power, 2014-09-12 22:06:48]

Committed patchset #1 (id:1) as 11304957eb3934100d364215085bf739430db2f2

[commit-bot: I haz the power, 2014-09-12 22:12:38]

Patchset 1 (id:??) landed as
https://crrev.com/7eff2e4b1a12584cd89f48ca3b9879faa37ebedc
Cr-Commit-Position: refs/heads/master@{#294675}