Pass --future flag correctly in v8.browsing_*-future benchmarks

AppendExtraBrowserArgs takes only one flag at a time. Fix
v8.browsing_desktop-future and v8.browsing_mobile-future to
call AppendExtraBrowserArgs twice to pass two flags.

Bug: 
Change-Id: I85bfe7b69be2164a26d77a9bc8bacb7272eff10f
Reviewed-on: https://chromium-review.googlesource.com/814437Reviewed-by: Ned Nguyen <nednguyen@google.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522472}

tools/perf/benchmarks/v8_browsing.py

@@ -224,8 +224,8 @@ class V8FutureDesktopBrowsingBenchmark(
 
   def SetExtraBrowserOptions(self, options):
     options.AppendExtraBrowserArgs(
-      '--enable-blink-features=BlinkRuntimeCallStats '
-      '--enable-features=V8VmFuture')
+      '--enable-blink-features=BlinkRuntimeCallStats')
+    options.AppendExtraBrowserArgs('--enable-features=V8VmFuture')
 
   @classmethod
   def Name(cls):
@@ -240,7 +240,8 @@ class V8FutureMobileBrowsingBenchmark(
 
   def SetExtraBrowserOptions(self, options):
     options.AppendExtraBrowserArgs(
-      '--enable-blink-features=BlinkRuntimeCallStats '
+      '--enable-blink-features=BlinkRuntimeCallStats')
+    options.AppendExtraBrowserArgs(
       '--enable-features=V8VmFuture')
 
   def GetExpectations(self):