c++11: Allow lambdas, with some restrictions.

Thanks to David Michael <dmichael@chromium.org> for condensing the discussion
thread to a recommendation!

BUG=none

Review URL: https://codereview.chromium.org/699283002

Cr-Commit-Position: refs/heads/master@{#302708}

chrome/browser/history/history_backend_unittest.cc

@@ -65,18 +65,6 @@ typedef base::Callback<void(const history::URLRow*,
                             const history::URLRow*)>
     SimulateNotificationCallback;
 
-// Comparison functions as to make it easier to check results of
-// GetFaviconBitmaps() and GetIconMappingsForPageURL().
-bool IconMappingLessThan(const history::IconMapping& a,
-                         const history::IconMapping& b) {
-  return a.icon_url < b.icon_url;
-}
-
-bool FaviconBitmapLessThan(const history::FaviconBitmap& a,
-                           const history::FaviconBitmap& b) {
-  return a.pixel_size.GetArea() < b.pixel_size.GetArea();
-}
-
 class HistoryClientMock : public history::HistoryClientFakeBookmarks {
  public:
   MOCK_METHOD0(BlockUntilBookmarksLoaded, void());
@@ -368,7 +356,9 @@ class HistoryBackendTest : public HistoryBackendTestBase {
       return false;
     }
     std::sort(icon_mappings->begin(), icon_mappings->end(),
-              IconMappingLessThan);
+              [](const history::IconMapping& a, const history::IconMapping& b) {
+      return a.icon_url < b.icon_url;
+    });
     return true;
   }
 
@@ -378,8 +368,11 @@ class HistoryBackendTest : public HistoryBackendTestBase {
                                std::vector<FaviconBitmap>* favicon_bitmaps) {
     if (!backend_->thumbnail_db_->GetFaviconBitmaps(icon_id, favicon_bitmaps))
       return false;
-    std::sort(favicon_bitmaps->begin(), favicon_bitmaps->end(),
-              FaviconBitmapLessThan);
+    std::sort(
+        favicon_bitmaps->begin(), favicon_bitmaps->end(),
+        [](const history::FaviconBitmap& a, const history::FaviconBitmap& b) {
+          return a.pixel_size.GetArea() < b.pixel_size.GetArea();
+        });
     return true;
   }
 

styleguide/c++/c++11.html

@@ -115,6 +115,23 @@ enum classes and regular enums.</td>
 <td>Recommended for new code. Existing uses of the <code>FINAL</code> macro will be <a href="https://crbug.com/417463">replaced throughout the codebase</a>. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/VTNZzizN0zo">Discussion thread</a></td>
 </tr>
 
+<tr>
+<td>Lambda Expressions</td>
+<td><code>[<i>captures</i>](<i>params</i>) -&gt; <i>ret</i> { <i>body</i> }</code></td>
+<td>Anonymous functions</td>
+<td><a href="http://en.cppreference.com/w/cpp/language/lambda">Lambda functions</a></td>
+<td>Do not bind or store lambdas; use <code>base::Bind</code> and
+<code>base::Callback</code> instead, because they offer protection against a
+large class of object lifetime mistakes. Don't use default captures
+(<code>[=]</code>, <code>[&amp;]</code> &ndash; <a
+  href="https://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Lambda_expressions">Google Style Guide</a>).
+Lambdas are typically useful as a parameter to methods or
+functions that will use them immediately, such as those in
+<code>&lt;algorithm&gt;</code>. <a
+  href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/D9UnnxBnciQ">Discussion
+  thread</a></td>
+</tr>
+
 <tr>
 <td>Local Types as Template Arguments</td>
 <td></td>
@@ -414,14 +431,6 @@ synthetic function such as a copy constructor</td>
 <td>Unclear how it will work with components</td>
 </tr>
 
-<tr>
-<td>Lambda Expressions</td>
-<td><code>[<i>captures</i>](<i>params</i>) -&gt; <i>ret</i> { <i>body</i> }</code></td>
-<td>Anonymous functions</td>
-<td><a href="http://en.cppreference.com/w/cpp/language/lambda">Lambda functions</a></td>
-<td>No default captures (<a href="https://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Lambda_expressions">Google Style Guide</a>). <a href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/D9UnnxBnciQ">Discussion thread</a></td>
-</tr>
-
 <tr>
 <td>Non-Static Class Member Initializers</td>
 <td>

ui/base/layout.cc

@@ -28,10 +28,6 @@ namespace ui {
 
 namespace {
 
-bool ScaleFactorComparator(const ScaleFactor& lhs, const ScaleFactor& rhs){
-  return GetScaleForScaleFactor(lhs) < GetScaleForScaleFactor(rhs);
-}
-
 std::vector<ScaleFactor>* g_supported_scale_factors = NULL;
 
 const float kScaleFactorScales[] = {1.0f, 1.0f, 1.25f, 1.33f, 1.4f, 1.5f, 1.8f,
@@ -49,7 +45,9 @@ void SetSupportedScaleFactors(
   g_supported_scale_factors = new std::vector<ScaleFactor>(scale_factors);
   std::sort(g_supported_scale_factors->begin(),
             g_supported_scale_factors->end(),
-            ScaleFactorComparator);
+            [](ScaleFactor lhs, ScaleFactor rhs) {
+    return GetScaleForScaleFactor(lhs) < GetScaleForScaleFactor(rhs);
+  });
 
   // Set ImageSkia's supported scales.
   std::vector<float> scales;