Add public doc for accessing C++ enums in Java.

docs/android_accessing_cpp_enums_in_java.md

+# Accessing C++ Enums In Java
+
+[TOC]
+
+## Introduction
+
+The traditional method involved extracting the enum values to a separate `_list.h` file so that we could then use gcc -E to generate a Java file (yes, we use the C preprocessor to codegen Java). The new method involves a Python script which analyzes the C++ enum and spits out the corresponding Java class. The enum needs to be annotated in a particular way. By default, the generated class name will be the same as the name of the enum. If all the names of the enum values are prefixed with the MACRO\_CASED\_ name of the enum those prefixes will be stripped from the Java version.

[agrieve, 2016/11/17 15:16:14]

let's not mention the previous way :P.

Also - please wrap lines to 80 characters

[estevenson, 2016/11/17 15:52:58]

Done.

+
+## Features
+* Customize the package name of the generated class using the `GENERATED_JAVA_ENUM_PACKAGE` directive (required)
+* Customize the class name using the `GENERATED_JAVA_CLASS_NAME_OVERRIDE` directive (optional)
+* Strip enum entry prefixes to make the generated classes less verbose using the `GENERATED_JAVA_PREFIX_TO_STRIP` directive (optional)
+* Supports [`@IntDef`](https://developer.android.com/reference/android/support/annotation/IntDef.html)
+* Copies comments that directly preceed enum entries into the generated Java class
+
+## Usage
+
+1. Add directives to your C++ enum
+
+    ```cpp

[estevenson, 2016/11/16 22:51:10]

I indented these blocks so that list continuation would work. Using
md_browser.py shows these blocks as formatted incorrectly while a few online md
browsers I tried format the blocks correctly. 

Any ideas on the correct way to do this? If not I can change after submitting if
it's wrong.

+    // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.chrome
+    // GENERATED_JAVA_CLASS_NAME_OVERRIDE: FooBar
+    // GENERATED_JAVA_PREFIX_TO_STRIP: BAR_
+    enum SomeEnum {
+      BAR_A,
+      BAR_B,
+      BAR_C = BAR_B,
+    };
+    ```
+
+2. Add a new build target
+
+    ```
+    import("//build/config/android/rules.gni")
+
+    java_cpp_enum("foo_generated_enum") {
+      sources = [
+        "base/android/native_foo_header.h",
+      ]
+    }
+    ```
+
+3. Add the new target to the desired android_library targets srcjar_deps:
+
+    ```
+    android_library("base_java") {
+      srcjar_deps = [
+        ":foo_generated_enum",
+      ]
+    }
+    ```
+
+4. The generated file `org/chromium/chrome/FooBar.java` would contain:
+
+    ```java
+    package org.chromium.chrome;
+
+    import android.support.annotation.IntDef;
+
+    import java.lang.annotation.Retention;
+    import java.lang.annotation.RetentionPolicy;
+
+    public class FooBar {
+      @IntDef({
+          A, B, C
+      })
+      @Retention(RetentionPolicy.SOURCE)
+      public @interface FooBarEnum {}
+      public static final int A = 0;
+      public static final int B = 1;
+      public static final int C = 1;
+    }
+    ```
+
+## Formatting Notes
+
+* Handling long package names:
+
+    ```
+    // GENERATED_JAVA_ENUM_PACKAGE: (
+    //   org.chromium.chrome.this.package.is.much.too.long.to.fit.on.a.single.line)
+    ```
+
+* Enum entries
+    * Single line enums should look like this:
+
+            // GENERATED_JAVA_ENUM_PACKAGE: org.foo
+            enum NotificationActionType { BUTTON, TEXT };
+
+    * Multi-line enums should have one enum entry per line, like this:
+
+            // GENERATED_JAVA_ENUM_PACKAGE: org.foo
+            enum NotificationActionType {
+              BUTTON,
+              TEXT
+            };
+
+    * Multi-line enum entries are allowed but should be formatted like this:
+
+            // GENERATED_JAVA_ENUM_PACKAGE: org.foo
+            enum NotificationActionType {
+              LongKeyNumberOne,
+              LongKeyNumberTwo,
+              ...
+              LongKeyNumberThree =
+                  LongKeyNumberOne | LongKeyNumberTwo | ... 
+            };
+
+* Preserving comments
+
+    ```cpp
+    // GENERATED_JAVA_ENUM_PACKAGE: org.chromium
+    enum CommentEnum {
+      // This comment will be preserved.
+      ONE,
+      TWO, // This comment will NOT be preserved.
+      THREE
+    }
+    ```
+
+    ```java
+    ...
+    public class CommentEnum {
+      ...
+      /**
+       * This comment will be preserved.
+       */
+      public static final int ONE = 0;
+      public static final int TWO = 1;
+      public static final int THREE = 2;
+    }
+    ```
+
+## Code
+* [Generator code](https://cs.chromium.org/chromium/src/build/android/gyp/java_cpp_enum.py?dr=C&sq=package:chromium) and [Tests](https://cs.chromium.org/chromium/src/build/android/gyp/java_cpp_enum_tests.py?dr=C&q=java_cpp_enum_tests&sq=package:chromium&l=1)
+* [GN template](https://cs.chromium.org/chromium/src/build/config/android/rules.gni?q=java_cpp_enum.py&sq=package:chromium&dr=C&l=458)