[foozzie] Fix detection of suppression delimiter

NOTRY=true
TBR=jkummerow@chromium.org,bmeurer@chromium.org

Bug: chromium:718739
Change-Id: I9b80a3098bf2c8d3a294e1908f058cabe1ed758c
Reviewed-on: https://chromium-review.googlesource.com/506092Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45322}

tools/foozzie/v8_suppressions.py

@@ -276,9 +276,14 @@ class V8Suppression(Suppression):
 
   def ignore_by_content(self, testcase):
     # Strip off test case preamble.
+    try:
       lines = testcase.splitlines()
       lines = lines[lines.index('print("js-mutation: start generated test case");'):]
       content = '\n'.join(lines)
+    except ValueError:
+      # Search the whole test case if preamble can't be found. E.g. older
+      # already minimized test cases might have dropped the delimiter line.
+      content = testcase
     for bug, exp in IGNORE_TEST_CASES.iteritems():
       if exp.search(content):
         return bug