devtools: Show clients where they can set breakpoints (#37667)

devtools clients query source actors to determine where the user can set
breakpoints in a source. there are two relevant requests here:
`getBreakableLines` controls which line numbers can be clicked in the
margin, and once a line number is clicked,
`getBreakpointPositionsCompressed` controls where to show breakpoint
buttons within that line.

this patch handles those requests by querying the [SpiderMonkey Debugger
API](https://firefox-source-docs.mozilla.org/js/Debugger/) for that
information:
- devtools sends its script thread a GetPossibleBreakpoints message for
the source’s
[`spidermonkey_id`](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.Source.html#id)
- the script thread fires a `getPossibleBreakpoints` event into its
debugger global
- the debugger script looks up the
[root](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.html#onnewscript-script-global)
[Debugger.Script](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.Script.html#getpossiblebreakpoints-query)
for that source, calls
[getPossibleBreakpoints()](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.Script.html#getpossiblebreakpoints-query),
and returns the result via
DebuggerGlobalScope#getPossibleBreakpointsResult()
- that method takes the pending result sender, and sends the result back
to devtools
- devtools massages the result into the format required by the request,
and replies to the client

as a result, users of the Firefox devtools client can now set
breakpoints, though they don’t have any effect.

Testing: this patch adds new devtools tests
Fixes: part of #36027

<img width="1433" height="1328" alt="image"
src="https://github.com/user-attachments/assets/f0cd31e0-742f-44d3-8c5d-ceedd9a2706d"
/>

---------

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: atbrakhi <atbrakhi@igalia.com>

components/shared/devtools/lib.rs

@@ -108,7 +108,7 @@ pub enum ScriptToDevtoolsControlMsg {
     TitleChanged(PipelineId, String),
 
     /// Get source information from script
-    CreateSourceActor(PipelineId, SourceInfo),
+    CreateSourceActor(IpcSender<DevtoolScriptControlMsg>, PipelineId, SourceInfo),
 
     UpdateSourceContent(PipelineId, String),
 }
@@ -284,6 +284,8 @@ pub enum DevtoolScriptControlMsg {
     SimulateColorScheme(PipelineId, Theme),
     /// Highlight the given DOM node
     HighlightDomNode(PipelineId, Option<String>),
+
+    GetPossibleBreakpoints(u32, IpcSender<Vec<RecommendedBreakpointLocation>>),
 }
 
 #[derive(Clone, Debug, Deserialize, Serialize)]
@@ -608,3 +610,12 @@ pub struct SourceInfo {
     pub content_type: Option<String>,
     pub spidermonkey_id: u32,
 }
+
+#[derive(Clone, Debug, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct RecommendedBreakpointLocation {
+    pub offset: u32,
+    pub line_number: u32,
+    pub column_number: u32,
+    pub is_step_start: bool,
+}