Add .gitignore files in build/ and third_party/

Having this, a lot of wasted bandwidth is avoided for projects
that use these directories as DEPS entries, since the top-level
Chromium .gitignore doesn't exist in those cases.

The src/ repo already contained 51 other .gitignore files anyway,
so it's hard to claim the top-level file has to be the only one.

BUG=690010

Review-Url: https://codereview.chromium.org/2710593005
Cr-Commit-Position: refs/heads/master@{#452167}
3 files changed