Change build-ctags.sh output file name from ".tags" to "tags"

Change build-ctags.sh output file name from ".tags" to "tags"

Two reasons for this change:
1. Default value of the tags option in Vim (that tells it
where to load tags from) is: "tags=./tags,./TAGS,tags,TAGS"
So now Vim should be able to pick up tags with default
settings when current dir is chromium/src
2. src/.gitignore already has "tags" in it, but not ".tags"
which, I think, indicates that people expect the tags file
name to not have a leading dot

BUG=none

Committed: https://crrev.com/fbc0ebe4967befb4643a3e286b0f8f376322dc07
Cr-Commit-Position: refs/heads/master@{#307539}

[servolk, 2014-12-09 19:14:50]

servolk@chromium.org changed reviewers:
+ jww@chromium.org

[servolk, 2014-12-09 19:15:36]

servolk@chromium.org changed reviewers:
+ thakis@chromium.org

[Nico, 2014-12-09 19:17:13]

lgtm if jww likes it

[jww, 2014-12-09 19:46:34]

Sure, lgtm

[servolk, 2014-12-09 19:47:21]

The CQ bit was checked by servolk@chromium.org

[commit-bot: I haz the power, 2014-12-09 19:48:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/791723002/1

[commit-bot: I haz the power, 2014-12-09 20:35:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-09 20:36:56]

Patchset 1 (id:??) landed as
https://crrev.com/fbc0ebe4967befb4643a3e286b0f8f376322dc07
Cr-Commit-Position: refs/heads/master@{#307539}