GitLab

For some USB drives, writes on Mac are much slower than using the old image
creator. This is most likely because the implementation in Chrome uses
/dev/disk# whereas the old implementation used /dev/rdisk# whenever
possible.

The main requirement is that writes are in multiples of the block size, but
since ImageWriter::WriteChunk always writes in multiples of
kMemoryAlignment (4096), it should be fine to always use the raw disk.

My brief tests using a Lexar JumpDrive 16GB and a 1.6GB image:
before: ~6.5 minutes to write, same to verify
after: ~5 minutes to write, ~1.5 minutes to verify

BUG=415891

Review URL: https://codereview.chromium.org/599853002

Cr-Commit-Position: refs/heads/master@{#296875}