[crosperf] Fix reading from cache.

The read-from-cache code needed to be updated to deal with the new
telemetry results format, as did the code for generating json
reports (for archiving in the database).

BUG=chromium:639106
TEST=Tested with crosperf reading from cache & generating json results.

Change-Id: I2c97e5b393583048cc74559d065f498c0f0675a1
Reviewed-on: https://chrome-internal-review.googlesource.com/278298
Commit-Ready: Caroline Tice <cmtice@google.com>
Tested-by: Caroline Tice <cmtice@google.com>
Reviewed-by: Yunlian Jiang <yunlian@google.com>
2 files changed