[Code health] Rename a value in ThreadType

Right now the FrameSequenceTracker::ThreadType has 3 values:
kMain
kCompositor
kSlowerThread

The kSlowerThread should just be kSlower, to be consistent with the
other two.

Bug: None
Change-Id: Ib4aa32b1313bdf72b1317f37f29193beab28622c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856728Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705361}

cc/metrics/frame_sequence_tracker.cc

@@ -38,7 +38,7 @@ constexpr int kMinFramesForThroughputMetric = 4;
 enum class ThreadType {
   kMain,
   kCompositor,
-  kSlowerThread,
+  kSlower,
 };
 
 constexpr int kBuiltinSequenceNum =
@@ -254,8 +254,7 @@ void FrameSequenceTracker::ReportMetrics() {
   }
   if (slower_throughput.has_value()) {
     ThroughputData::ReportHistogram(
-        type_, "SlowerThread",
-        GetIndexForMetric(ThreadType::kSlowerThread, type_),
+        type_, "SlowerThread", GetIndexForMetric(ThreadType::kSlower, type_),
         slower_throughput.value());
   }