Null check ExecutionContext in NavigatorUA::MaybeRecordMetrics().

We're getting crashes that imply it can be null here (though it
can't be null in NavigatorUAData::getHighEntropyValues(), apparently).

Bug: 1134994
Change-Id: Ie361625dc6df4d25592bfe167a82119a7b00167d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450289Reviewed-by: Asanka Herath <asanka@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814241}

third_party/blink/renderer/core/frame/navigator_ua.cc

@@ -43,6 +43,10 @@ void NavigatorUA::MaybeRecordMetrics(const NavigatorUAData& ua_data) {
     return;
   }
 
+  ExecutionContext* context = GetUAExecutionContext();
+  if (!context)
+    return;
+
   // Only instrument low-entropy fields here. The other fields are
   // instrumented separately in NavigatorUAData::getHighEntropyValues().
   IdentifiableTokenBuilder token_builder;
@@ -55,7 +59,6 @@ void NavigatorUA::MaybeRecordMetrics(const NavigatorUAData& ua_data) {
     if (brand->hasVersion())
       token_builder.AddAtomic(brand->version().Utf8());
   }
-  ExecutionContext* context = GetUAExecutionContext();
   IdentifiabilityMetricBuilder(context->UkmSourceID())
       .Set(identifiable_surface, token_builder.GetToken())
       .Record(context->UkmRecorder());