webkit: Remove webgraphicscontext3d_impl.* entries that were left behind.

I forgot removing these when doing:
https://chromium.googlesource.com/chromium/src/+/13f5f5e58cf2b2bf9a5ad9918a37294d38bb2c4c

BUG=338338
TEST=compiles
TBR=piman@chromium.org

Review URL: https://codereview.chromium.org/791903004

Cr-Commit-Position: refs/heads/master@{#309391}

webkit/common/gpu/webkit_gpu.gyp

@@ -42,8 +42,6 @@
             'context_provider_web_context.h',
             'grcontext_for_webgraphicscontext3d.cc',
             'grcontext_for_webgraphicscontext3d.h',
-            'webgraphicscontext3d_impl.cc',
-            'webgraphicscontext3d_impl.h',
             'webgraphicscontext3d_in_process_command_buffer_impl.cc',
             'webgraphicscontext3d_in_process_command_buffer_impl.h',
           ],