Clarify that enum classes include explicit underlying types.

Also use that in one place to forward-declare an enum, to make sure it actually
works. No behavior change.

BUG=none

Review URL: https://codereview.chromium.org/678333004

Cr-Commit-Position: refs/heads/master@{#301766}

3 files changed, 9 insertions, 7 deletions

diff --git a/styleguide/c++/c++11.html b/styleguide/c++/c++11.html
index b62f3c0..c0274d9 100644
--- a/styleguide/c++/c++11.html
+++ b/styleguide/c++/c++11.html
@@ -94,12 +94,14 @@ decltype specifier</a></td>
 </tr>
 
 <tr>
-<td>Enumerated Type Classes</td>
-<td><code>enum class <i>classname</i></code></td>
+<td>Enumerated Type Classes and Enum Bases</td>
+<td><code>enum class <i>classname</i></code><br>
+    <code>enum class <i>classname</i> : <i>base-type</i></code><br>
+    <code>enum <i>enumname</i> : <i>base-type</i></code></td>
 <td>Provide enums as full classes, with no implicit
-conversion to booleans or integers</td>
-<td><a href="http://stackoverflow.com/questions/6936030/do-we-really-need-enum-class-in-c11">
-enum-class</a></td>
+conversion to booleans or integers. Provide an explicit underlying type for
+enum classes and regular enums.</td>
+<td><a href="http://www.stroustrup.com/C++11FAQ.html#enum">enum-class</a></td>
 <td>Enum classes are still enums and follow enum naming rules
 (which means SHOUTY_CASE in the <a href="http://www.chromium.org/developers/coding-style#Naming">current style guide</a>).
 <a href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/Q5WmkAImanc">Discussion thread</a></td>
diff --git a/ui/base/layout.h b/ui/base/layout.h
index eaaa926..ab83878 100644
--- a/ui/base/layout.h
+++ b/ui/base/layout.h
@@ -18,7 +18,7 @@ namespace ui {
 // SCALE_FACTOR_NONE is used for density independent resources such as
 // string, html/js files or an image that can be used for any scale factors
 // (such as wallpapers).
-enum ScaleFactor {
+enum ScaleFactor : int {
   SCALE_FACTOR_NONE = 0,
   SCALE_FACTOR_100P,
   SCALE_FACTOR_125P,
diff --git a/ui/base/resource/data_pack.h b/ui/base/resource/data_pack.h
index 928c855..b7d833c 100644
--- a/ui/base/resource/data_pack.h
+++ b/ui/base/resource/data_pack.h
@@ -16,7 +16,6 @@
 #include "base/files/memory_mapped_file.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string_piece.h"
-#include "ui/base/layout.h"
 #include "ui/base/resource/resource_handle.h"
 #include "ui/base/ui_base_export.h"
 
@@ -26,6 +25,7 @@ class RefCountedStaticMemory;
 }
 
 namespace ui {
+enum ScaleFactor : int;
 
 class UI_BASE_EXPORT DataPack : public ResourceHandle {
  public: