Fix failing custom filters tests

Add a return false case in StyleCustomFilterProgram::isLoaded for when m_vertexShader and m_fragmentShader are null. This can happen when ResourceFetcher refuses to create them because the page is being torn down.

Change the hasPendingShaders flag to hasNewCustomFIlterProgram. Before, the hasPendingShaders = false flag was keeping loadPendingShaders from proceeding. loadPendingShaders should always look up new StyleCustomFilterPrograms in the StyleCustomFilterProgramCache to see if there is an matching one that's already loaded the same resources.

BUG=280554, 243291

Review URL: https://codereview.chromium.org/26895003

git-svn-id: svn://svn.chromium.org/blink/trunk@160155 bbb929c8-8fbe-4397-9dbb-9b2b20218538