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

docs/android_accessing_cpp_enums_in_java.md

+# Accessing C++ Enums In Java
+
+[TOC]
+
+## Introduction
+
+Accessing C++ enums in Java is implemented via 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.
+
+## 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
+    // 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.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)