Disable text selection in line number columns of |blink_tool.py pretty-diff|

Copying diff content is useful, and line numbers should not be copied
together.

Change-Id: I396c6bffee03683228df31b6dfa753da8e1a6d94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2029469Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790716}

third_party/blink/tools/blinkpy/common/pretty_diff.py

@@ -55,10 +55,11 @@ td { white-space: pre-wrap; font-size: 14px; }
   color: #666;
   padding: 0 0.5em;
   text-align: right;
+  user-select: none;
   vertical-align: top;
   width: 94px;
 }
-.emptylineno { box-sizing: border-box; width: 94px; }
+.emptylineno { box-sizing: border-box;  user-select: none; width: 94px; }
 .code { border-left: 1px solid #ddd; word-break: break-all; }
 .del { background: #ffeeee; }
 .del.strong { background: #ffcaca; }