Migrate benchmark_json_merge in webkitpy/benchmark_runner to Python 3

https://bugs.webkit.org/show_bug.cgi?id=260307

Reviewed by Dewei Zhu.

Imports reduce from functools since it is no longer a built-in function in Python 3.
Uses Python 3 list comprehension syntax to add the object keys.

* Tools/Scripts/webkitpy/benchmark_runner/benchmark_json_merge.py:
(deepAppend):

Canonical link: https://commits.webkit.org/267216@main

Tools/Scripts/webkitpy/benchmark_runner/benchmark_json_merge.py

@@ -24,6 +24,8 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 # THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+from functools import reduce
+
 
 def deepAppend(value1, value2, currentKey=None):
     if type(value1) != type(value2):
@@ -32,7 +34,7 @@ def deepAppend(value1, value2, currentKey=None):
         return value1 + value2
 
     result = {}
-    for key in (value1.keys() + value2.keys()):
+    for key in [*value1.keys(), *value2.keys()]:
         if key not in result:
             result[key] = deepAppend(value1[key], value2[key], key)
     return result