Fix api,operation,rendering,draw:vertex_attributes,basic:* (#702)
Fix api,operation,rendering,draw:vertex_attributes,basic:* (#702)

This patch fixes all the bugs in the operation test
pi,operation,rendering,draw:vertex_attributes,basic:* to make them
pass with latest Chromium.
1. Only use 14 locations for user-defined inter-stage variables.
2. Set writeMask to 0 when there is no color as the fragment output.
gpuweb/cts,gpuweb/cts,gpuweb/cts