Add Apple copyright notice to HTMLSrcsetParser

When porting the original srcset parser from WebKit, the code was split from
HTMLParserIdioms into a separate file - HTMLSrcsetParser.
The Apple copyright notice of that code was mistakenly not copied to the new file.
This CL corrects this.

BUG=
TBR=pdr

Review URL: https://codereview.chromium.org/305653007

git-svn-id: svn://svn.chromium.org/blink/trunk@174994 bbb929c8-8fbe-4397-9dbb-9b2b20218538