With a local rjsmin installed, the bundled rjsmin in //third_party/WebKit/Source/devtools/scripts/build can pick up the local version's native module, which can be older and not compatible with some recent "modern" js code in devtools, like the one added in TargetManager.js in https://codereview.chromium.org/2729623006.

Causes errors like:
1:15:02 [8874/24545] ACTION //content/browser/devtools:devtools_resources_grit(//build/toolchain/android:android_arm)
11:15:02 FAILED: gen/content/browser/devtools/devtools_resources_stamp.d.stamp gen/content/browser/devtools/grit/devtools_resources.h gen/content/browser/devtools/devtools_resources.pak gen/content/browser/devtools/grit/devtools_resources_map.cc gen/content/browser/devtools/grit/devtools_resources_map.h 
11:15:02 python ../../tools/grit/grit.py -i gen/devtools/devtools_resources.grd build -o gen/content/browser/devtools --depdir . --depfile gen/content/browser/devtools/devtools_resources_stamp.d --write-only-new=1 --depend-on-stamp -D _chromium -E CHROMIUM_BUILD=chromium -E ANDROID_JAVA_TAGGED_ONLY=true -t android -D SHARED_INTERMEDIATE_DIR=gen -f ../../tools/gritsettings/resource_ids --assert-file-list=obj/content/browser/devtools/devtools_resources_expected_outputs.txt --js-minifier /home/jenkins/shared_ws/opera_work_master-next/chromium/src/third_party/closure_compiler/js_minify.py\ --closure_args\ extra_annotation_name=attribute\ extra_annotation_name=demo\ extra_annotation_name=element\ extra_annotation_name=group\ extra_annotation_name=hero\ extra_annotation_name=homepage\ extra_annotation_name=status\ extra_annotation_name=submodule\ source_map_format=V3\ compilation_level=WHITESPACE_ONLY\ language_in=ECMASCRIPT_NEXT\ language_out=ECMASCRIPT6\ jscomp_off=duplicate\ jscomp_off=misplacedTypeAnnotation
11:15:02 Minification failed for gen/devtools/inspector.js
11:15:02 /tmp/tmpUCzDAL.js:3404: ERROR - Parse error. ':' expected
11:15:02 addAttribute(key,value){this._attributes[key.toLowerCase()]=value;}};SDK.Cookie.Type={Request:0,Response:1};;SDK.ProfileNode=class{constructor(callFrame){this.callFrame=callFrame;this.callUID=`${this.callFrame.functionName}@${this.callFrame.scriptId}:${this.callFrame.lineNumber}`;this.self=0;this.total=0;this.id=0;this.parent=null;this.children=[];}
11:15:02                                                                                                                                                                                                  ^
[snip]
11:15:02 109 error(s), 0 warning(s)
11:15:02 
11:15:02 Traceback (most recent call last):
11:15:02   File "/home/jenkins/shared_ws/opera_work_master-next/chromium/src/third_party/closure_compiler/js_minify.py", line 46, in <module>
11:15:02     result = Minify(sys.stdin.read())
11:15:02   File "/home/jenkins/shared_ws/opera_work_master-next/chromium/src/third_party/closure_compiler/js_minify.py", line 38, in Minify
11:15:02     return result
11:15:02   File "/usr/lib/python2.7/tempfile.py", line 427, in __exit__
11:15:02     self.close()
11:15:02   File "/usr/lib/python2.7/tempfile.py", line 418, in close
11:15:02     self.unlink(self.name)
11:15:02 OSError: [Errno 2] No such file or directory: '/tmp/tmpj0Vn7_.js'
11:15:02 
11:15:02 Error processing node <?xml version="1.0" encoding="UTF-8"?>
11:15:02 <include file="inspector.js" name="INSPECTOR_JS" type="BINDATA" />

(the tmp file error at the end is a distraction, real error is that the input file to the closure minifier was already broken by rjsmin beforehand)

https://codereview.chromium.org/2800063003/ should fix the one instance where a patched python rjsmin would sometimes pickup an unpatched system native version, but there are other imported  rjsmin.py files all over the tree that may or may not cause issues:
./v8/src/inspector/build/rjsmin.py
./third_party/trace-viewer/third_party/tvcm/third_party/rjsmin/rjsmin.py
./third_party/WebKit/Source/devtools/scripts/build/rjsmin.py  <-- I'm fixing this one
./third_party/catapult/third_party/py_vulcanize/third_party/rjsmin/rjsmin.py

Re #3: Does Issue 712745 represent such a problem?

The assigned owner "tsniatowski@opera.com" is not able to receive e-mails, please re-triage.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot