Update blink-gen-property-trees bot configuration to use new flag

In https://crrev.com/612683 the blink-gen-property-trees flag was
changed from --enable-blink-gen-property-trees to
--enable-blink-features=BlinkGenPropertyTrees. This patch updates the
bot configuration to use the new flag.

Bug: 910859
Change-Id: I75bd3b947cf6fe5e8ec43a34a60e9cb7d61152c5
Reviewed-on: https://chromium-review.googlesource.com/c/1359131Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613250}

testing/buildbot/chromium.fyi.json

@@ -4288,7 +4288,7 @@
         "args": [
           "--num-retries=3",
           "--debug",
-          "--additional-driver-flag=--enable-blink-gen-property-trees"
+          "--additional-driver-flag=--enable-blink-features=BlinkGenPropertyTrees"
         ],
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {

testing/buildbot/test_suite_exceptions.pyl

@@ -1318,7 +1318,7 @@
       'linux-blink-gen-property-trees': {
         'args': [
           '--debug',
-          '--additional-driver-flag=--enable-blink-gen-property-trees',
+          '--additional-driver-flag=--enable-blink-features=BlinkGenPropertyTrees',
         ],
       },
       'linux-blink-heap-incremental-marking': {