chromium: Update all channels to latest versions

Overview of the updated versions:

stable: 49.0.2623.87 -> 49.0.2623.110
beta:   50.0.2661.26 -> 50.0.2661.49
dev:    50.0.2661.18 -> 51.0.2693.2

Most notably, this includes a series of urgent security fixes:

 * CVE-2016-1646: Out-of-bounds read in V8. Credit to Wen Xu from
                  Tencent KeenLab.
 * CVE-2016-1647: Use-after-free in Navigation. Credit to anonymous.
 * CVE-2016-1648: Use-after-free in Extensions. Credit to anonymous.
 * CVE-2016-1649: Buffer overflow in libANGLE. Credit to lokihardt
                  working with HP's Zero Day Initiative / Pwn2Own.
 * CVE-2016-1650: Denial of service in PageCaptureSaveAsMHTMLFunction

The official release announcement with details about these fixes can be
found here:

http://googlechromereleases.blogspot.de/2016/03/stable-channel-update_24.html

Beta and stable could be also affected, although I didn't do a detailed
check whether that's the case.

As this introduces Chromium 51 as the dev version, I had to make the
following changes to make it build:

 * libexif got removed, so let's do that on our end as well.
   See https://codereview.chromium.org/1803883002 for details.
 * Chromium doesn't seem to compile with our version of libpng, so let's
   resort to the bundled libpng for now.
 * site_engagement_ui.cc uses isnan outside of std namespace, so
   we're fixing that in postPatch using sed.

I have successfully built all versions on i686-linux and x86_64-linux
and tested it using the VM tests.

Test reports can be found at the following evaluation of my Hydra:

https://headcounter.org/hydra/eval/314584



Thanks to @grahamc for reporting this.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Reported-by: Graham Christensen <graham@grahamc.com>
Fixes: #14299