Add PLATFORM_EXPORT to MediaStreamComponent::AudioSourceProviderImpl

On non-Windows, PLATFORM_EXPORT expands to a visibility attribute,
which is inherited by nested classes. On Windows, it's expanded to
a dllexport/dllimport attribute, which is not inherited.

When using Clang on Windows, MediaStreamComponent's dtor got
inlined, which destroys m_sourceProvider, whose destructor got inlined,
and tried to reference provideInput for the vtable, but failed to link
since it wasn't in the right dll. Exporting the inner class fixes this.

BUG=82385
TEST=build blink_web.dll with Clang on Windows in Release mode

Review URL: https://codereview.chromium.org/343763002

git-svn-id: svn://svn.chromium.org/blink/trunk@176468 bbb929c8-8fbe-4397-9dbb-9b2b20218538