Implement retry behavior for several leveldb env methods in mojo env.

The existing ChromiumEnv class retries several methods on failure for a
bit, this CL adds that same retry logic to the new MojoEnv leveldb env.

Bug: 586194
Change-Id: I6d3805ed8e0db6cbd1363a85b087ee0753000cfb
Reviewed-on: https://chromium-review.googlesource.com/528294
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Michael Nordman <michaeln@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478720}
3 files changed