Try to fix bitmaptools declaration in gn_migration.gypi.

Try to fix bitmaptools declaration in gn_migration.gypi.

I think the analyze generator is getting confused by
a mismatch between bitmaptools and bitmaptools#host,
but am not sure, so this is a speculative fix.

TBR=brettw@chromium.org

Committed: https://crrev.com/379e4987df67c0525a6f6950c10c461e2cd5115f
Cr-Commit-Position: refs/heads/master@{#318634}

[Dirk Pranke, 2015-03-01 22:17:32]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-03-01 22:18:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/970713002/1

[commit-bot: I haz the power, 2015-03-01 23:18:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-01 23:18:49]

Patchset 1 (id:??) landed as
https://crrev.com/379e4987df67c0525a6f6950c10c461e2cd5115f
Cr-Commit-Position: refs/heads/master@{#318634}