metrics: minor cleanup of platform_field_trials.h

The trailing ';' isn't necessary when you have {}, but since I'm
changing this may as well use = default;

BUG=none
TEST=none

Change-Id: Ibc96ce7471e82b46ed1291e3350993c5dcbe7762
Reviewed-on: https://chromium-review.googlesource.com/857741
Commit-Queue: Ilya Sherman <[email protected]>
Reviewed-by: Ilya Sherman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#528140}
diff --git a/components/variations/platform_field_trials.h b/components/variations/platform_field_trials.h
index bba99b8..3a7f642 100644
--- a/components/variations/platform_field_trials.h
+++ b/components/variations/platform_field_trials.h
@@ -13,8 +13,8 @@
 // WebView make use through their corresponding subclasses.
 class PlatformFieldTrials {
  public:
-  PlatformFieldTrials(){};
-  virtual ~PlatformFieldTrials(){};
+  PlatformFieldTrials() = default;
+  virtual ~PlatformFieldTrials() = default;
 
   // Set up field trials for a specific platform.
   virtual void SetupFieldTrials() = 0;