[Reland] Narrow silk pageset to the most plausible content

This removes a few pages that were duplicative or not mobile-focused enough to
meet the silk bar. Then, "why" descriptions were added to explain why the page
is on the pageset. Finallly, it moves a few pages from file urls to
jankfree.org/silk in order to have them point at image resources.

Original review at https://codereview.chromium.org/140653014

TBR=tonyg
BUG=340777

Review URL: https://codereview.chromium.org/184663007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254357 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254357