[PDF OCR] Record "Accessibility.PDF.HasAccessibleText" only once

Add a flag to indicate that a PDF accessibility tree was created once.
It helps that one UMA metric, "Accessibility.PDF.HasAccessibleText",
gets recorded only once when a PDF is opened.

(cherry picked from commit 690f47b)

AX-Relnotes: n/a.
Bug: 1458740
Change-Id: I8af2cd8af5e66fefd0b9be94a30fd85a31eda97c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4655048
Commit-Queue: Kyungjun Lee <kyungjunlee@google.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1164203}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4678964
Auto-Submit: Kyungjun Lee <kyungjunlee@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/branch-heads/5845@{#422}
Cr-Branched-From: 5a5dff6-refs/heads/main@{#1160321}

components/pdf/renderer/pdf_accessibility_tree.cc

@@ -1784,8 +1784,15 @@ void PdfAccessibilityTree::UnserializeNodes() {
   render_accessibility->SetPluginTreeSource(this);
   nodes_.clear();
 
-  base::UmaHistogramBoolean("Accessibility.PDF.HasAccessibleText",
-                            did_get_a_text_run_);
+  if (!did_unserialize_nodes_once_) {
+    // If the user turns on PDF OCR after opening a PDF, its PDF a11y tree gets
+    // created again. `did_unserialize_nodes_once_` helps to determine whether
+    // it's first time to create a PDF a11y tree. When a PDF is opened, the UMA
+    // metric, "Accessibility.PDF.HasAccessibleText", needs be recorded once.
+    did_unserialize_nodes_once_ = true;
+    base::UmaHistogramBoolean("Accessibility.PDF.HasAccessibleText",
+                              did_get_a_text_run_);
+  }
 }
 
 #if BUILDFLAG(ENABLE_SCREEN_AI_SERVICE)

components/pdf/renderer/pdf_accessibility_tree.h

@@ -259,6 +259,7 @@ class PdfAccessibilityTree : public content::PluginAXTreeSource,
   uint32_t next_page_index_ = 0;
 
   bool did_get_a_text_run_ = false;
+  bool did_unserialize_nodes_once_ = false;
 
 #if BUILDFLAG(ENABLE_SCREEN_AI_SERVICE)
   // The status node contains a notification message for the user.