Also pretty-print negative LayoutUnit values with lldb.

Also avoid scientific notation;
see https://codereview.chromium.org/1202773002 for a similar change
to the gdb script.

Change-Id: I07ddccd91ddefc1f57abbaabaaecb5e786a052e4
Reviewed-on: https://chromium-review.googlesource.com/894062Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533093}

third_party/WebKit/Tools/lldb/lldb_webkit.py

@@ -182,7 +182,7 @@ class WebCoreLayoutUnitProvider:
         self.valobj = valobj
 
     def to_string(self):
-        return "%gpx" % (self.valobj.GetChildMemberWithName('value_').GetValueAsUnsigned(0) / 64.0)
+        return "%.14gpx" % (self.valobj.GetChildMemberWithName('value_').GetValueAsSigned(0) / 64.0)
 
 
 class WebCoreLayoutSizeProvider: