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

Review URL: https://codereview.chromium.org/791723002

Cr-Commit-Position: refs/heads/master@{#307539}