Cache --whitelisted-extension-id in SimpleFeature.

During startup, CommandLine::HasSwitch(kWhitelistedExtensionID) is
called ~1000 times by SimpleFeature. It is the most common switch
lookup, the second being kAppsGalleryUpdateUrl at ~300 times.

Caching this avoids allocating and copying the string each time.

BUG=471976,476061

Committed: https://crrev.com/16ca320592ce7ef6cd36500da6e670cc645781cd
Cr-Commit-Position: refs/heads/master@{#324583}

Review URL: https://codereview.chromium.org/1047943002

Cr-Commit-Position: refs/heads/master@{#324821}
8 files changed