DevTools: gray out linenumbers for the unmapped source code lines

This patch:
- grays out linenumbers for the unmapped source code lines
- disabled setting breakpoint on these lines

These are cosmetic changes - currently, breakpoints will
never break if set on the line without mapping.

BUG=479733
R=paulirish

Review-Url: https://codereview.chromium.org/2120603002
Cr-Commit-Position: refs/heads/master@{#403950}
2 files changed