clang/win: Enable oilpan plugin on 64-bit builds again.

This reverts https://codereview.chromium.org/1129103005/ , which turned
them off.  Now what all [blink-gc] warnings are fixed, we can turn them
on again on the warnings-as-errors 64-bit builds.

BUG=486571,467287
TBR=hans

Review URL: https://codereview.chromium.org/1131333003

Cr-Commit-Position: refs/heads/master@{#330313}

build/common.gypi

@@ -2236,12 +2236,6 @@
         ],
       }],
 
-      ['OS=="win" and target_arch=="x64"', {
-        # TODO(thakis): Enable on x64 once all warnings are fixed.
-        # http://crbug.com/486571
-        'blink_gc_plugin%': 0,
-      }],
-
       # On valgrind bots, override the optimizer settings so we don't inline too
       # much and make the stacks harder to figure out.
       #