Update tools/gn/README.md.

Update tools/gn/README.md.

This patch updates the GN README to reflect the current
state of the migration, and also fixes the links to the
GN docs so that they work correctly (working around
https://code.google.com/p/gitiles/issues/detail?id=82).

R=andybons@chromium.org
BUG=

Committed: https://crrev.com/c96f78f835dffc09a3235ebbca7a98d7882b6bd1
Cr-Commit-Position: refs/heads/master@{#352994}

[Dirk Pranke, 2015-10-07 20:42:00]

Patchset #1 (id:1) has been deleted

[Dirk Pranke, 2015-10-07 20:43:09]

The downside to this approach to working around the gitiles bug is that if
someone happens to look at the README on a branch, the links will take you off
the branch and back to master.

In practice, I suspect this is not likely to happen often, so this is probably
the lesser of two evils.

[Bons, 2015-10-07 22:22:34]

lgtm

[Dirk Pranke, 2015-10-08 00:43:52]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-10-08 00:45:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1393863003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1393863003/20001

[commit-bot: I haz the power, 2015-10-08 01:00:35]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-10-08 01:04:42]

Patchset 1 (id:??) landed as
https://crrev.com/c96f78f835dffc09a3235ebbca7a98d7882b6bd1
Cr-Commit-Position: refs/heads/master@{#352994}