Conditional support for webp main page images

Webp is currently supported by Chrome and Opera. The image are still
on the big size, but about half the size. Used cwebp -preset photo.
jpg     webp    filename
432K    344K    aaretal.jpg
424K    144K    acueducto.jpg
256K    52K     alpamayo.jpg
212K    44K     concert-hall.jpg
164K    92K     farm-poster.jpg
476K    204K    ferronor.jpg
304K    96K     golden-gate.jpg
356K    120K    gorges-du-tarn.jpg
416K    152K    holzbrucke.jpg
380K    136K    kasumi.jpg
476K    212K    mabodalen.jpg
112K    44K     nepean-river.jpg
328K    112K    ruhrtalbruecke.jpg
312K    212K    taburiente.jpg
784K    160K    turtle.jpg
5.4M    2.1M    total

Change-Id: I61ee7037847e587d3d4f08d4e7464e092aca6b72
Update nginx mime.types from upstream

Change-Id: I4c03316b08e48dea80a372effc7a4e2c76d0cbe2
Merge "Conditional support for webp main page images"
Merge "Update nginx mime.types from upstream"