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

[Dor1s]

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

[vsk]

LGTM. Thanks!

llvm/trunk/tools/llvm-cov/SourceCoverageViewHTML.cpp

 th, td {
   vertical-align: top;
   padding: 2px 8px;
   border-collapse: collapse;
   border-right: solid 1px #eee;
   border-left: solid 1px #eee;
   text-align: left;
 }
+td pre {
+  display: inline-block;
+}
 td:first-child {
   border-left: none;
 }
 td:last-child {
   border-right: none;
 }
 tr:hover {
   background-color: #f0f0f0;