[AdTagging] Add documentation for ad tagging

Bug: 851533
Change-Id: I6cc6def8e7def7b6231dfb7e735a66b596b90b36
Reviewed-on: https://chromium-review.googlesource.com/c/1371964Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Commit-Queue: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615564}

docs/ad_tagging.md

+# Ad Tagging in Chromium
+
+Chromium is able to detect some ads and the resources they load in the
+browser. This enables the browser to measure the size, performance, and count of
+ads displayed to our users. It also allows the browser to intervene on the
+user’s behalf when ads run counter to the user’s interest (e.g., by using
+excessive resources or by engaging in [abusive
+behavior](https://support.google.com/webtools/answer/7347327).
+
+The ad detection infrastructure is called Ad Tagging. Ad Tagging works by
+matching resource requests against a filter list (see how the list is
+[generated](https://chromium.googlesource.com/chromium/src.git/+/master/components/subresource_filter/FILTER_LIST_GENERATION.md))
+to determine if they’re ad requests. Any requests matching the filter are tagged
+as ads. Further, requests (and some DOM elements such as iframes) made on behalf
+of previously tagged scripts are also tagged as ads by the
+[AdTracker](https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/frame/ad_tracker.h). An
+iframe will be marked as an ad iframe if its url matches the filter list, if
+tagged script is involved in the creation of the iframe, or if its parent frame
+is an ad iframe. The main frame of a page will never be tagged as an ad. Any
+request made within an ad iframe is considered an ad resource request.
+
+## Basic Architecture
+
+### Subresource Filter
+The [Subresource
+Filter](https://chromium.googlesource.com/chromium/src.git/+/master/components/subresource_filter/README.md)
+loads the filter list and matches urls against it. The list is distributed via
+the component updater. This same list and component is used for blocking ads on
+abusive sites and those that violate the Better Ads Standard.
+
+Each subresource request in the render process is processed by the subresource
+filter before the request is sent. If it matches the list, the ResourceRequest
+is tagged as an ad.
+
+Each subframe navigation request is processed in the browser process by the
+SubresourceFilter and if the final URL in the navigation matches the list then
+the render process is told (via the SubresourceFilterAgent) and the iframe is
+marked as an ad iframe.
+
+### AdTracker
+The
+[AdTracker](https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/frame/ad_tracker.h)
+keeps track of each script subresource that is considered an ad. Any time that a
+new resource is requested or an iframe is created, V8's stack is scanned and if
+any of the known ad scripts are in the stack then the resource request or iframe
+is considered an ad.
+