Enable fractional Mouse type pointerEvent by default

Enable factional mouse type pointer event by default

Intent to implement and ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/ssfd-_V26Ls

Bug: 773813
Change-Id: Ie8010d27b4c0a6b43fec7cfab04e153dce21536d
Reviewed-on: https://chromium-review.googlesource.com/753511Reviewed-by: Rick Byers <rbyers@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515072}

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -440,7 +440,7 @@
     },
     {
       name: "FractionalMouseTypePointerEvent",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "FractionalScrollOffsets",