Drop 'const' declaration from courgette unittest.

---
../../courgette/label_manager_unittest.cc:41:3: error: 'const' type qualifier on return type has no effect [-Werror,-Wignored-qualifiers]
  const size_t LabelCount() const { return labels_.size(); };
  ^~~~~~
1 error generated.
---

BUG=567505
TBR=huangs@chromium.org
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1501413004

Cr-Commit-Position: refs/heads/master@{#363756}

courgette/label_manager_unittest.cc

@@ -38,7 +38,7 @@ class TestRvaVisitor : public LabelManager::RvaVisitor {
 // Test version of LabelManager: Expose data to test implementation.
 class TestLabelManager : public LabelManager {
  public:
-  const size_t LabelCount() const { return labels_.size(); };
+  size_t LabelCount() const { return labels_.size(); };
 };
 
 void CheckLabelManagerContent(TestLabelManager* label_manager,