[Coverage] Update CSS to make HTML reports copy-paste friendly.

Summary:
This minor change allows to copy snippets from HTML reports so they
will be pasted in the following format:
%LineNumber%\t%HitCount%\t%CodeLine%

rather then being split onto multiple lines. To see this in action, try copy
pasting from https://chromium-coverage.appspot.com/reports/560344/linux/chromium/src/third_party/zlib/compress.c.html

Requested in https://bugs.chromium.org/p/chromium/issues/detail?id=845571

Reviewers: vsk

Reviewed By: vsk

Subscribers: llvm-commits, morehouse, kcc

Differential Revision: https://reviews.llvm.org/D47231

llvm-svn: 333034