Plugin Power Saver: Replace all "plug-in" with "plugin" for consistency.

Passed chrome-ui-review: https://groups.google.com/a/google.com/d/msg/chrome-ui-review/FwuQ2TWc5n8/H3PSuV4L_cAJ

BUG=467165

TBR=tsepez
(comment changes only in render_messages)

NOPRESUBMIT=true
(legacy LOG messages)

Review URL: https://codereview.chromium.org/1007443004

Cr-Original-Commit-Position: refs/heads/master@{#320782}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e86b2982a90cc87afbeb8599cc54c274c482b0ec

common.gypi

@@ -581,7 +581,7 @@
       # Use experimental lld linker instead of the platform's default linker.
       'use_lld%': 0,
 
-      # Enable plug-in installation by default.
+      # Enable plugin installation by default.
       'enable_plugin_installation%': 1,
 
       # Specifies whether to use canvas_skia.cc in place of platform