Reland "depot_tools: Ensure Python is bootstrapped before reading python3_bin_reldir.txt"

This reverts commit b2bfd55f.

Reason for revert: not responsible for crbug.com/1025659

Original change's description:
> Revert "depot_tools: Ensure Python is bootstrapped before reading python3_bin_reldir.txt"
> 
> This reverts commit bf26b167.
> 
> Reason for revert: spec revert for https://bugs.chromium.org/p/chromium/issues/detail?id=1025659
> 
> Original change's description:
> > depot_tools: Ensure Python is bootstrapped before reading python3_bin_reldir.txt
> > 
> > Bug: 1017812
> > Change-Id: I7013e5445ff5917fab813a54fed416d9770d2684
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1913269
> > Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> > Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> 
> TBR=dpranke@chromium.org,ehmaldonado@chromium.org,apolito@google.com
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Bug: 1017812
> Change-Id: I8e10894a2e835c20ece7d555c21f6ce1f76528a5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1922949
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: John Budorick <jbudorick@chromium.org>

Bug: 1017812
Change-Id: I251c6b1a3e3a7b9ac9d21dabe745466c668a8246
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1924494Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

vpython

@@ -34,13 +34,20 @@ then
 fi
 
 
-MYPATH=$(dirname "${BASH_SOURCE[0]}")
+base_dir=$(dirname "$0")
 
-source "$MYPATH/cipd_bin_setup.sh"
+source "$base_dir/cipd_bin_setup.sh"
 cipd_bin_setup &> /dev/null
 
-base_dir=$(dirname "$0")
-PYTHON_BIN_RELDIR="$(cat $base_dir/python_bin_reldir.txt | xargs echo)"
-PATH="$PYTHON_BIN_RELDIR":"$PYTHON_BIN_RELDIR/Scripts":"$PATH"
+# If Python bootstrapping is not disabled, make sure Python has been
+# bootstrapped and add it to the front of PATH.
+if [[ $MINGW != 0 && $DEPOT_TOOLS_BOOTSTRAP_PYTHON3 != 0 ]]; then
+  if [[ ! -e $base_dir/python_bin_reldir.txt ]]; then
+    source "$base_dir/bootstrap_python3"
+    bootstrap_python3
+  fi
+  PYTHON_BIN_RELDIR="$base_dir/$(cat $base_dir/python_bin_reldir.txt | xargs echo)"
+  PATH="$PYTHON_BIN_RELDIR":"$PYTHON_BIN_RELDIR/Scripts":"$PATH"
+fi
 
-exec "$MYPATH/.cipd_bin/vpython" "$@"
+exec "$base_dir/.cipd_bin/vpython" "$@"

vpython3

@@ -34,13 +34,20 @@ then
 fi
 
 
-MYPATH=$(dirname "${BASH_SOURCE[0]}")
+base_dir=$(dirname "$0")
 
-source "$MYPATH/cipd_bin_setup.sh"
+source "$base_dir/cipd_bin_setup.sh"
 cipd_bin_setup &> /dev/null
 
-base_dir=$(dirname "$0")
-PYTHON3_BIN_RELDIR="$(cat $base_dir/python3_bin_reldir.txt | xargs echo)"
-PATH="$PYTHON3_BIN_RELDIR":"$PYTHON3_BIN_RELDIR/Scripts":"$PATH"
+# If Python bootstrapping is not disabled, make sure Python has been
+# bootstrapped and add it to the front of PATH.
+if [[ $MINGW != 0 && $DEPOT_TOOLS_BOOTSTRAP_PYTHON3 != 0 ]]; then
+  if [[ ! -e $base_dir/python3_bin_reldir.txt ]]; then
+    source "$base_dir/bootstrap_python3"
+    bootstrap_python3
+  fi
+  PYTHON3_BIN_RELDIR="$base_dir/$(cat $base_dir/python3_bin_reldir.txt | xargs echo)"
+  PATH="$PYTHON3_BIN_RELDIR":"$PYTHON3_BIN_RELDIR/Scripts":"$PATH"
+fi
 
-exec "$MYPATH/.cipd_bin/vpython3" "$@"
+exec "$base_dir/.cipd_bin/vpython3" "$@"