Add //tools/gdb/OWNERS

Change-Id: I34e8872d1011d143810a2f6b53a238e517785bf2
Reviewed-on: https://chromium-review.googlesource.com/1255045Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595844}

tools/gdb/OWNERS

+jbroman@chromium.org
+thomasanderson@chromium.org
+tikuta@chromium.org