Merge 105121 - Numerous fixes to audio/video buffered resource loading.

Merge 105121 - Numerous fixes to audio/video buffered resource loading.

This patch fixes a few related issues:
  1) Default loading strategy is now threshold-then-defer
  2) Specify more reasonable default bitrate/playback rate values
  3) Use a minimum buffer window size to prevent underflows on low bitrate content
  4) Remember bitrate/playback rate values between resource loaders

The default loading strategy of read-then-defer had a negative impact on initial latency as we were constantly deferring/undeferring the connection during the time when we need data the fastest.

While this change does result in loading a pinch more data for preload=metadata scenarios, it vastly improves the common preload=auto scenario.

BUG=99775
TEST=test_shell_tests

Review URL: http://codereview.chromium.org/8224028
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=105963