Add src/extensions to All target in gyp

Use the same condition as chromium_builder_tests.

The targets are already in all_gn, and run on the trybots
via the chromium_builder_tests target. Putting them in All
just helps developers spot compile errors locally, before
they're caught by the bots.

Review URL: https://codereview.chromium.org/1053563004

Cr-Original-Commit-Position: refs/heads/master@{#324739}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f42e70932de69422378f3fef25df8afc4f2bb8e1

all.gyp

@@ -113,6 +113,8 @@
             '../cc/blink/cc_blink_tests.gyp:*',
             '../cc/cc_tests.gyp:*',
             '../device/usb/usb.gyp:*',
+            '../extensions/extensions.gyp:*',
+            '../extensions/extensions_tests.gyp:*',
             '../gin/gin.gyp:*',
             '../gpu/gpu.gyp:*',
             '../gpu/tools/tools.gyp:*',