Add multiselect to alerts page
Add multiselect to alerts page

Shift + click gets you multiple selection

BUG=skia:5455

Change-Id: I37dcddafd8e0c108685d5bb2b5535ab0f1f2642b
Reviewed-on: https://skia-review.googlesource.com/5336
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot