[Devtools] Parse source maps lazyly

On some webpages, devtools may run out of memory simply due to the size of the source maps.

BUG=631389
R=lushnikov@chromium.org

Review-Url: https://codereview.chromium.org/2371133003
Cr-Commit-Position: refs/heads/master@{#422452}
6 files changed