Add myself as dedicated .clang-format owner.

As requested by jam in https://chromium-review.googlesource.com/c/chromium/src/+/1542137/2#message-6bf92e08011129e70b560c3571119b8f531c7188

(I considered adding file://styleguide/c++/OWNERS instead, but this file
is also used for java and javascript and there's no toplevel styleguide/OWNERS
file yet and this file almost never changes anyways.)

Bug: none
Change-Id: I42fd67ecc4bf4b70a234c04b599bd57575e188c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1549201
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646937}

OWNERS

@@ -3,6 +3,7 @@
 # Eng reviewer. Please reach out before adding new top-level directories.
 file://ENG_REVIEW_OWNERS
 
+per-file .clang-format=thakis@chromium.org
 per-file .eslintrc.js=file://tools/web_dev_style/OWNERS
 per-file .gitattributes=*
 per-file .gitignore=*