Allow trace-viewer to use exec_script in GN.
Allow trace-viewer to use exec_script in GN.

Trace-viewer currently maintains two copies of its input files. This
causes duplicate work each time we add a file. I'd like to share a
single .gypi file between GYP and GN but need to use exec_script
in order to do so.

This CL adds an exeception to allow trace-viewer to use exec_script
in its BUILD.gn file.

Review URL: https://codereview.chromium.org/1160793005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#332892}

hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Just-D/chromium-1,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,Chilledheart/chromium,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk