Add importorder to Java styleguide.

styleguide/java/java.md

 # Chromium Java style guide
 
 _For other languages, please see the [Chromium style
 guides](https://chromium.googlesource.com/chromium/src/+/master/styleguide/styleguide.md)._
 
 Chromium follows the [Android Open Source style
 guide](http://source.android.com/source/code-style.html) unless an exception
 is listed below.
 
 A checkout should give you clang-format to automatically format Java code.
@@ skipping 106 matching lines @@
 Example use of `DCHECK_IS_ON`:
 
 ```java
 if (org.chromium.base.BuildConfig.DCHECK_IS_ON) {
    if (!someCallWithSideEffects()) {
      throw new AssertionError("assert description");
    }
 }
 ```
 
+### Import Order
+
+* Static imports go before other imports.
+* Each import group must be separated by an empty line.
+
+This is the order of the import groups:
+
+1. android
+1. com (except com.google.android.apps.chrome)
+1. dalvik
+1. junit
+1. org
+1. com.google.android.apps.chrome
+1. org.chromium
+1. java
+1. javax
+
+This is enforced by the
+[Chromium Checkstyle configuration](../../tools/android/checkstyle/chromium-style-5.0.xml)
+under the ImportOrder module.
+
 ## Location
 
 "Top level directories" are defined as directories with a GN file, such as
 [//base](https://chromium.googlesource.com/chromium/src/+/master/base/)
 and
 [//content](https://chromium.googlesource.com/chromium/src/+/master/content/),
 Chromium Java should live in a directory named
 `<top level directory>/android/java`, with a package name
 `org.chromium.<top level directory>`.  Each top level directory's Java should
 build into a distinct JAR that honors the abstraction specified in a native
 [checkdeps](https://chromium.googlesource.com/chromium/buildtools/+/master/checkdeps/checkdeps.py)
 (e.g. `org.chromium.base` does not import `org.chromium.content`).  The full
 path of any java file should contain the complete package name.
 
 For example, top level directory `//base` might contain a file named
 `base/android/java/org/chromium/base/Class.java`. This would get compiled into a
 `chromium_base.jar` (final JAR name TBD).
 
 `org.chromium.chrome.browser.foo.Class` would live in
 `chrome/android/java/org/chromium/chrome/browser/foo/Class.java`.
 
 New `<top level directory>/android` directories should have an `OWNERS` file
 much like
 [//base/android/OWNERS](https://chromium.googlesource.com/chromium/src/+/master/base/android/OWNERS).
 
 ## Miscellany
 
 * Use UTF-8 file encodings and LF line endings.