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This CL includes 2 event fixes that were originally part of
CL [1] to fix similar event-related issues.

* event.h
    - "As() const" always called back into itself, causing an
      infinite loop.  The intention was to call the non-const
      As() function.
* event.cc
    - Xlib modifies the event data when parsing some events,
      but we never do, so let us parse the event before Xlib.
* gen_xproto.py
    - Ensure the extension is present before creating extension
      events.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2253223/

BUG=1098194, 1098204
R=thestig

Change-Id: I414acd9c138a92fbddd09da705127f1d7f60b552
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261177
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781615}