IndexedDB: Eliminated false detection of database corruption.

Our leveldb_env::IsCorruption check falsely reported an InvalidArgument Status
as a corrupted status. Now that leveldb correctly returns a corruption status
(as of v1.18) we can remove this function and fix this bug.

BUG=429209

Review URL: https://codereview.chromium.org/740513002

Cr-Commit-Position: refs/heads/master@{#304714}
6 files changed