Make v8 inspector not crash

It was attempting to load InjectedScript, etc.
with the wrong path and hitting a CHECK().
I moved InjectedScript and DebugScript into
the v8_inspector directory and fixed their loading.

I also broke the dependency from v8_inspector
onto engine/platform by moving the horrible
Platform::loadResource hack out of PlatformImpl
into a new file just for v8_inspector.

In this process I also found some (broken!)
code for showing the broken image icon which
if we ever hit would just have crashed, so
I removed that code as well.

BUG=435243
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/776743002