The prettify symbol 'PR' at the top level window object may conflict
with a name created by the optimized GWT compiler, resulting in
conflicting ideas about what the PR name really is and causing
crashes in particular browsers.
Move prettify into its own iframe, where it has its own top level
window object, and thus has its own isolated namespace.
Change-Id: I30523854067eb33250810e9d2f7e23b4c691541f
Signed-off-by: Shawn O. Pearce <sop@google.com>
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