Add code review policy document.

docs/code_reviews.md

+# Code Reviews
+
+Code reviews are a central part of developing high-quality code for Chromium.
+All changes must be reviewed.
+
+The bigger patch-upload-and-land process is covered in more detail the
+[contributing code](https://www.chromium.org/developers/contributing-code)
+page.
+
+# Code review policies
+
+Ideally the reviewer is someone who is familiar with the area of code you are
+touching. If you have doubts, look at the git blame for the file and the OWNERS
+files (see below).
+
+Any committer can review code, but there must be at least one owner for each
+directory you are touching. If you have multiple reviewers, make it clear in
+the message you send requesting review what you expect from each reviewer.
+Otherwise people might assume their input is not required or waste time with
+redundant reviews.
+
+## Expectations for all reviewers
+
+  * Aim to provide some kind of actionable response within 24 hours of receipt
+    (not counting weekends and holidays). This doesn't mean you have to have
+    done a complete review, but you should be able to give some initial
+    feedback, request more time, or suggest another reviewer.
+
+  * It can be nice to indicate if you're away in your name in the code review
+    tool. If you do this, indicate when you'll be back.
+
+  * Don't generally discourage people from sending you code reviews. This
+    includes writing a blanket ("slow") after your name in the review tool.
+
+## OWNERS files
+
+In various directories there are files named "OWNERS" that list the email
+addresses of people qualified to review changes in that directory. You must
+get a positive review from an owner of each directory your change touches.
+
+Owners files are recursive, so each file also applies to its subdirectories. If
+an owner file contains the text "set noparent" it will stop owner propagation
+from parent directories.
+
+Tip: The "git cl owners" command can help find owners.
+
+While owners must approve all patches, any committer can contribute to the
+review. In some directories the owners can be overloaded or there might be
+people not listed as owners who are more familiar with the low-level code in
+question. In these cases it's common to request a low-level review from an
+appropriate person, and then request a high-level owner review once that's
+complete. As always, be clear what you expect of each reviewer to avoid
+duplicated work.
+
+### Expectations for owners
+
+  * OWNERS files should contain only people actively reviewing code in that
+    directory.
+
+  * If you aren't doing code reviews in a directory, remove yourself. Don't
+    try to discourage people from sending reviews in comments in the OWNERS
+    file. This includes writing "slow" or "emeritus" after your name.
+
+  * If the code review load is unsustainable, work to expand the number of
+    owners.
+
+## TBR ("To Be Reviewed")
+
+"TBR" is our mechanism for post-commit review. It should be used rarely and
+only in cases where a review is unnecessary or as described below. The most
+common use of TBR is to revert patches that broke the build.
+
+TBR does not mean "no review." A reviewer on a TBRed on a change should still
+review the change. If there comments after landing the author is still
+obligated to address them in a followup patch.
+
+Do not use TBR just because a change is urgent or the reviewer is being slow.
+Work to contact a reviewer who can do your review in a timely manner.
+
+To send a change TBR, annotate the description and send email like normal.
+Otherwise the reviewer won't know to review the patch.
+
+  * Add the reviewer's email address in the code review tool's reviewer field
+    like normal.
+
+  * Add a line "TBR=<reviewer's email>" to the bottom of the change list
+    description.
+
+  * Push the "send mail" button.
+
+### TBR-ing certain types of mechanical changes
+
+Sometimes you might do something that affects many callers in different
+directories. For example, adding a parameter to a common function in //base.
+If the updates to the callers is mechanical, you can:
+
+  * Get a normal owner of the lower-level directory (in this example, //base)
+    you're changing to do a proper review of those changes.
+
+  * Get _somebody_ to review the downstream changes. This is often the same
+    person from the previous step but could be somebody else.
+
+  * Add the owners of the affected downstream directories as TBR.
+
+This process ensures that all code is reviewed prior to checkin and that the
+concept of the change is reviewed by a qualified person, but you don't have to
+track down many individual owners for trivial changes to their directories.
+
+### TBR-ing documentation updates
+
+You can TBR documentation updates. Documentation means markdown files, text
+documents, and high-level comments in code. At finer levels of detail, comments
+in source files become more like code and should be reviewed normally (not
+using TBR). Non-TBR-able stuff includes things like function contracts and most
+comments inside functions.
+
+  * Use your best judgement. If you're changing something very important,
+    tricky, or something you may not be very familiar with, ask for the code
+    review up-front.
+
+  * Don't TBR changes to policy documents like the style guide or this document.
+
+  * Don't mix unrelated documentation updates with code changes.
+
+  * Be sure to actually send out the email for the code review. If you get one,
+    please actually read the changes. Even the best writers have editors and
+    checking prose for clarity is much faster than checking code for
+    correctness.