WebUI cleanup: Rename use_vulcanize GN flag to optimize_webui

Vulcanize tool has been replaced with polymer-bundler. But also several
other optimization tools are invoked when this flag is used
(polymer-css-build, crisper, uglify), therefore the more generic
"optimize_webui" name seems more appropriate. This is addressing an
already existing TODO in the code.

Bug: 731881
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I0d25b1306141ef01f4d40a0cb9c0b79cccb6837f
Reviewed-on: https://chromium-review.googlesource.com/663215
Commit-Queue: Demetrios Papadopoulos <[email protected]>
Reviewed-by: Dan Beam (no longer on Chrome) <[email protected]>
Cr-Commit-Position: refs/heads/master@{#502639}
diff --git a/docs/optimizing_web_uis.md b/docs/optimizing_web_uis.md
index 8a669fb..5b1b62f 100644
--- a/docs/optimizing_web_uis.md
+++ b/docs/optimizing_web_uis.md
@@ -6,7 +6,7 @@
 GN args:
 
 ```
-use_vulcanize = true
+optimize_webui = true
 is_debug = false
 ```