Update guidelines for using STL and base in Blink

This follows the discussion which happened on platform-architecture-dev:
https://groups.google.com/a/chromium.org/forum/#!topic/platform-architecture-dev/wuLAMcSvVqs/discussion

Change-Id: I1b65537eadb3ddd5dd3e1a981a3ae5b3e9432e34
Reviewed-on: https://chromium-review.googlesource.com/1094478
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569383}

styleguide/c++/blink-c++.md

@@ -47,13 +47,10 @@ FrameLoader::FrameLoader(LocalFrame* frame)
 }
 ```
 
-## Prefer WTF types over STL types
+## Prefer WTF types over STL and base types
 
-Outside of `//third_party/blink/common`, Blink should use WTF types. STL string
-and container types should only be used at the boundary to interoperate with
-'//base', `//third_party/blink/common`, and other Chromium-side code.
-Similarly, Blink should prefer `KURL` over `GURL` and `SecurityOrigin` over
-`url::Origin`.
+See [Blink readme](../../third_party/blink/renderer/README.md#Type-dependencies)
+for more details on Blink directories and their type usage.
 
 **Good:**
 ```c++

third_party/blink/renderer/README.md

@@ -127,16 +127,33 @@ See [this diagram](https://docs.google.com/document/d/1yYei-V76q3Mb-5LeJfNUMitmj
 
 ### Type dependencies
 
-`core/`, `modules/`, `bindings/`, `platform/` and `controller/` can use `std::` types and
-types defined in Chromium. The philosophy is that we should
-share as much code between Chromium and Blink as possible.
-
-However, there are a couple of types that really need to be optimized
-for Blink's workload (e.g., `Vector`, `HashTable`, `Bind`, `AtomicString`).
-These types are defined in `platform/wtf`. If there is an equivalent in
-`platform/wtf`, Blink must use the type in `platform/wtf` instead of the type
-defined in Chromium. For example, Blink should not use `std::vector`
-(except places where a conversion between `std::vector` and `WTF::Vector` is needed).
+Member variables of the following types are strongly discouraged in Blink:
+  - STL strings and containers. Use `WTF::String` and WTF containers instead.
+  - `GURL` and `url::Origin`. Use `KURL` and `SecurityOrigin` respectively.
+  - Any `//base` type which has a matching type in `platform/wtf`. The number of
+  duplicated types between WTF and base is continuously shrinking,
+  but always look at WTF first.
+
+The types above could only be used at the boundary to interoperate
+with `//base`, `//services`, `//third_party/blink/common` and other
+Chromium-side or third-party code. It is also allowed to use local variables
+of these types when convenient, as long as the result is not stored
+in a member variable.
+For example, calling an utility function on an `std::string` which came
+from `//net` and then converting to `WTF::String` to store in a field
+is allowed.
+
+We try to share as much code between Chromium and Blink as possible,
+so the number of these types should go down. However, some types
+really need to be optimized for Blink's workload (e.g., `Vector`,
+`HashTable`, `AtomicString`).
+
+Exceptions to this rule:
+  - Code in `//third_party/blink/common` and `//third_party/blink/public/common`
+  also runs in the browser process, and should use STL and base instead of WTF.
+  - Selected types in `public/platform` and `public/web`,
+  whole purpose of which is conversion between WTF and STL,
+  for example `WebString` or `WebVector`.
 
 To prevent use of random types, we control allowed types by whitelisting
 them in DEPS and a [presubmit script](../tools/audit_non_blink_usage.py).