[Blink] [PowerProfiler] Support accuracy level attribute for power data provider.

Blink side changes in devtools protocol and front_end to utilize the accuracy level
attribute from power data provider.

BUG=384299
NOTRY=true

Review URL: https://codereview.chromium.org/336713005

git-svn-id: svn://svn.chromium.org/blink/trunk@176966 bbb929c8-8fbe-4397-9dbb-9b2b20218538