remove skip tests after fixing v8:7961,v8:7958

remove skip for fast/js/string-prototype-properties.html and fast/js/date-proto-generic-invocation.html

Bug: v8:7961,v8:7958
Change-Id: Ib71d4084c850824b73c834997683d446f7b62781
Reviewed-on: https://chromium-review.googlesource.com/1212702Reviewed-by: Jungshik Shin <jshin@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589723}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2541,13 +2541,6 @@ crbug.com/591500 [ Win10 ] virtual/threaded/printing/quirks-percentage-height.ht
 crbug.com/591500 [ Win10 ] virtual/threaded/printing/standards-percentage-heights.html [ Failure ]
 crbug.com/591500 [ Win10 ] virtual/threaded/printing/subframes-percentage-height.html [ Failure ]
 
-
-# TODO(ftang,jshin) Remove once v8 is rolled to include a fix for v8/7958.
-crbug.com/v8/7958 fast/js/string-prototype-properties.html [ Skip ]
-
-# TODO(ftang) Remove once v8 is rolled to include a fix for v8/7961.
-crbug.com/v8/7961 fast/js/date-proto-generic-invocation.html [ Skip ]
-
 # TODO(chrishtr) uncomment ones marked crbug.com/591500 after fixing crbug.com/665259.
 crbug.com/591500 [ Win10 ] printing/fixed-positioned-headers-and-footers-absolute-covering-some-pages.html [ Failure ]
 crbug.com/591500 [ Win10 ] virtual/threaded/printing/fixed-positioned-headers-and-footers-absolute-covering-some-pages.html [ Failure ]