[blink]: Annotate StringImpl::createStatic leak for LeakSanitizer.

Adding the annotation WTF_ANNOTATE_SCOPED_MEMORY_LEAK in StringImpl::createStatic to ensure leak sanitizer doesn't report this as a leak.
This patch just handles Leak Sanitizer only. The changes for Heap Checker will be done in a follow up patch.

BUG=268258
TEST=Running content_unittests with Leak Sanitizer enabled.
Output of Leak Sanitizer before the fix: http://pastebin.com/niDgRNAQ
Output of Leak Sanitizer after the fix: http://pastebin.com/v6hF43Lj

Review URL: https://chromiumcodereview.appspot.com/23901002

git-svn-id: svn://svn.chromium.org/blink/trunk@157315 bbb929c8-8fbe-4397-9dbb-9b2b20218538