Initialize AudioContext on constructor to increase currentTime in real-time.

The AudioContext is not initialized to prevent unefficiant running of audio device
before any node is created in the context. However, it is mismatched with the specification.
According to the specification, currentTime must be increased when currentTime of context.

"This is a time in seconds which starts at zero when the context is created and increases in real-time."

TEST=manually tested that currentTime progresses by creating  an AudioNode a short time after creating the context and verified that currentTime was not zero.

BUG=377683

Review URL: https://codereview.chromium.org/297313003

git-svn-id: svn://svn.chromium.org/blink/trunk@175026 bbb929c8-8fbe-4397-9dbb-9b2b20218538