Use correct process lock for WaveShaperProcessor

The WaveShaperProcessor should inherit the m_processLock from the
parent class instead of creating its own.  The parent's m_processLock
correctly synchronizes access and processing.

Added a simple test from the bug report.  Manually tested to fail on
Chrome 35, but passes with this CL.

TEST=waveshaper-364379.html
BUG=364379

Review URL: https://codereview.chromium.org/264933002

git-svn-id: svn://svn.chromium.org/blink/trunk@173229 bbb929c8-8fbe-4397-9dbb-9b2b20218538