Reduce resolution of performance.now.

Reduce resolution of performance.now.
https://bugs.webkit.org/show_bug.cgi?id=146531
rdar://problem/20116796

Reviewed by Simon Fraser.

Source/WebCore:

Test: http/tests/misc/webtiming-resolution.html

* page/Performance.cpp:
(WebCore::Performance::now):
Floor the time returned by performance.now to the nearest 5 microseconds.

LayoutTests:

* http/tests/misc/webtiming-resolution-expected.txt: Added.
* http/tests/misc/webtiming-resolution.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@186208 268f45cc-cd09-0410-ab3c-d52691b4dbfc