Add warning for default disabled categories to caution the user before enabling.
Fix failing performace tests

Track contents drawing function is draw_() instead of drawTrackContents_().

Call to invalid function drawTrackContents_() is causing failure of performance tests:
tracing.tracks.drawing_container_perf_test.draw_softwareCanvas_1
tracing.tracks.drawing_container_perf_test.draw_softwareCanvas_10
tracing.tracks.drawing_container_perf_test.draw_softwareCanvas_100

drawTrackContents_() was renamed to draw_() as part of revision 1544 - Add collapsing to most tracks in the timeline

[1] https://code.google.com/p/trace-viewer/source/diff?spec=svn1544&r=1544&format=side&path=/trunk/trace_viewer/tracing/tracks/drawing_container.html

R=dsinclair@chromium.org
Scrollbar not working in timing mode

On clicking scroll bar of 'timing mode', thread times window is shifting to
right side and user is unable to scroll down. We are assuming that every hit
in timing mode is on the canvas area itself and moving to setting the interest
range value including that the range right is being selected. Do the interest
range actions only if the mousedown hit point is in the canvas rect.

BUG=549
Merge pull request #5 from kphanee/fix_perf_tests

Fix failing performace tests
Merge pull request #6 from kphanee/scroll_timing_mode

Scrollbar not working in timing mode
Merge pull request #7 from kphanee/disabled_category_warning

Add caution warning for user for default disabled categories