MSE-in-Workers: Drop dependency on HTMLME for recording counters

Upcoming worker-context-hosted MSE code paths will need to be distanced
from relying directly upon the main-thread-hosted HTMLMediaElement.

This change breaks one such strong dependency in MSE implementation by
using the already-known execution context in MediaSource and
SourceBuffer directly to record deprecation and usecounters, instead of
consulting to use the UseCounter context of the attached media element.

Also includes a necessary set of braces to fix a pre-existing style
error flagged this time by presubmit.

BUG=878133

Change-Id: I74b3ba0e9e11f866ffc1ec08376e045cee31efbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382678
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Will Cassella <cassew@google.com>
Cr-Commit-Position: refs/heads/master@{#803621}

third_party/blink/renderer/modules/mediasource/media_source.cc

@@ -606,7 +606,7 @@ void MediaSource::DurationChangeAlgorithm(double new_duration,
     }
 
     Deprecation::CountDeprecation(
-        attached_element_->GetExecutionContext(),
+        GetExecutionContext(),
         WebFeature::kMediaSourceDurationTruncatingBuffered);
     // See also deprecated remove(new duration, old duration) behavior below.
   }
@@ -625,9 +625,10 @@ void MediaSource::DurationChangeAlgorithm(double new_duration,
     // Deprecated behavior: if the new duration is less than old duration,
     // then call remove(new duration, old duration) on all all objects in
     // sourceBuffers.
-    for (unsigned i = 0; i < source_buffers_->length(); ++i)
+    for (unsigned i = 0; i < source_buffers_->length(); ++i) {
       source_buffers_->item(i)->remove(new_duration, old_duration,
                                        ASSERT_NO_EXCEPTION);
+    }
   }
 
   // 5. If a user agent is unable to partially render audio frames or text cues

third_party/blink/renderer/modules/mediasource/source_buffer.cc

@@ -416,9 +416,8 @@ void SourceBuffer::abort(ExceptionState& exception_state) {
       return;
     }
 
-    Deprecation::CountDeprecation(
-        source_->MediaElement()->GetExecutionContext(),
-        WebFeature::kMediaSourceAbortRemove);
+    Deprecation::CountDeprecation(GetExecutionContext(),
+                                  WebFeature::kMediaSourceAbortRemove);
     CancelRemove();
   }
 
@@ -1131,7 +1130,7 @@ void SourceBuffer::NotifyParseWarning(const ParseWarning warning) {
       // TODO(wolenetz): Use the data to scope additional work. See
       // https://crbug.com/739931.
       UseCounter::Count(
-          source_->MediaElement()->GetDocument(),
+          GetExecutionContext(),
           WebFeature::kMediaSourceKeyframeTimeGreaterThanDependant);
       break;
     case WebSourceBufferClient::kMuxedSequenceMode:
@@ -1140,7 +1139,7 @@ void SourceBuffer::NotifyParseWarning(const ParseWarning warning) {
       // SourceBuffer, at Media.OriginUrl.MSE.MuxedSequenceModeSourceBuffer.
       // TODO(wolenetz): Use the data to scope additional work. See
       // https://crbug.com/737757.
-      UseCounter::Count(source_->MediaElement()->GetDocument(),
+      UseCounter::Count(GetExecutionContext(),
                         WebFeature::kMediaSourceMuxedSequenceMode);
       break;
     case WebSourceBufferClient::kGroupEndTimestampDecreaseWithinMediaSegment:
@@ -1150,7 +1149,7 @@ void SourceBuffer::NotifyParseWarning(const ParseWarning warning) {
       // https://crbug.com/920853 and
       // https://github.com/w3c/media-source/issues/203.
       UseCounter::Count(
-          source_->MediaElement()->GetDocument(),
+          GetExecutionContext(),
           WebFeature::kMediaSourceGroupEndTimestampDecreaseWithinMediaSegment);
       break;
   }